Clerk vs Auth0 vs WorkOS: Real Authentication Cost for an AI SaaS at 100,000 MAU in 2026

The short answer: these three meter authentication on genuinely different units, and the title's "100,000 MAU" framing only directly applies to two of them. WorkOS's AuthKit user management is confirmed free up to 1 million Monthly Active Users — at 100,000 MAU with no enterprise SSO connections, WorkOS costs $0. Clerk bills Monthly Retained Users (MRU), a narrower unit than MAU (a user must return at least once to count), using a confirmed formula of $25/month base plus $0.02 per MRU from 50,001–100,000 and $0.018/MRU above that: at a worst-case 100,000 MRU (treating every MAU as retained), Clerk costs $1,025/month, and real Clerk cost at the same 100,000 raw MAU is likely lower, since not every active user returns to count as "retained." Auth0's self-serve B2C pricing effectively cannot reach 100,000 MAU at all — its published tiers top out around 50,000 MAU before requiring an unpublished Enterprise quote, making its true cost at this scale QUOTE-ONLY / UNKNOWN.

MAU versus MRU: not the same unit, and this article does not treat them as interchangeable

Clerk bills Monthly Retained Users, defined as a user who returns at least one day after initially signing up — not simply anyone who was active during the month. Auth0 and WorkOS both bill on raw Monthly Active Users. Because MRU is a subset of MAU by definition, a product with a high one-time-signup, low-return-visit pattern will have meaningfully fewer MRU than MAU in a given month, while a product where users log in repeatedly will have the two figures converge. This article does not know your specific retained-user ratio, so it shows two things separately: a worst-case normalization that treats 100,000 MAU as if every single one were also retained (100,000 MRU), which is the ceiling on what Clerk could cost at that raw user count, and an explicit note that real Clerk cost at 100,000 MAU can be lower — in some cases substantially lower — once accounting for users who sign up once and never return.

What each vendor bills, and on what unit

Clerk (OFFICIAL, Clerk's pricing page, confirmed rate card as of the current pricing documentation). Bills in Monthly Retained Users (MRU). Hobby: free, 50,000 MRU limit per app. Pro: $25/month ($20 billed annually), 50,000 MRU included, then $0.02 per additional MRU from 50,001 to 100,000, and $0.018 per MRU from 100,001 to 1,000,000 (volume discounts step down further above that, reported toward roughly $0.012/MRU at 10 million-plus). Business: $300/month ($250 annual), same MRU ladder as Pro, plus additional features. The formula: cost = $25 + ($0.02 × MRU in the 50,001–100,000 band) + ($0.018 × MRU above 100,000). At a worst-case 100,000 MRU, this is confirmed at $1,025/month; at 1,000,000 MRU, $17,225/month. An older, widely-repeated figure of roughly $800 and $5,500 at these two volumes does not follow from Clerk's current published rate card and should not be used.

Auth0 (OFFICIAL, Auth0/Okta Customer Identity Cloud pricing, with a real self-serve ceiling). Bills in discrete MAU tiers, not a flat per-MAU rate — landing between tiers bills at the next tier up. Free: up to 25,000 MAU, including, as of a February 2026 B2B upgrade, inbound SCIM and self-service SSO with one enterprise connection. B2C Essentials: $35/month at a low MAU ceiling, climbing through discrete tiers with overage at roughly $0.07/MAU once the tier's base limit is exceeded, self-serving up to roughly 50,000 MAU at $3,500/month before the published ceiling ends. Exceeding that published ceiling — including the 100,000 MAU this article's title specifies — becomes an unpublished Enterprise quote, not a predictable next tier. A separate B2B product line (Organizations) prices differently by connection count: $800/month at 500 MAU with 5 Enterprise Connections included, rising to $2,400/month at 10,000 MAU.

WorkOS (OFFICIAL, workos.com/pricing). AuthKit (user management): the first 1,000,000 Monthly Active Users are free, confirmed directly on WorkOS's own pricing page — "a user is considered active if they perform any user action (sign up, sign in, or profile update) within the calendar month." Beyond 1 million MAU: $2,500/month per additional million. Enterprise SSO and Directory Sync (SCIM) are billed separately, per connection, in volume tiers: $125/connection for the first 15, $100/connection for the 16th–30th, $80/connection for the 31st–50th, $65/connection for the 51st–100th, and $50/connection for the 101st–200th (custom pricing beyond 200). A connection represents one enterprise customer's relationship with WorkOS via their identity provider, independent of how many end users that customer has. Custom domain: $99/month. Audit log event retention: $99/month per million events stored. SIEM log streaming: $125/month per connection.

Cost at 100,000 MAU (or MRU, where that's the billed unit)

Formula: varies by vendor's own billing unit, as described above.

The gap between WorkOS's $0 and Clerk's $1,025 ceiling at the same raw user count is not a pricing error — WorkOS deliberately gives away basic authentication at this scale, betting that revenue comes from enterprise SSO connections once a B2B customer needs them, while Clerk monetizes retained usage directly from a much lower free threshold.

Why Auth0 doesn't have a clean answer at 100,000 MAU

This is a genuine structural gap in Auth0's self-serve pricing, not a research oversight. Auth0's B2C self-serve path is reported to top out around 50,000 MAU before requiring a sales conversation and an unpublished Enterprise quote. A team budgeting Auth0 for an AI SaaS product expecting to reach 100,000 MAU should plan for that negotiation well before hitting the number, not assume a predictable next-tier price exists to extrapolate from.

The per-connection versus per-user philosophy

This is the single most consequential structural difference in this comparison, not a minor billing detail. WorkOS prices enterprise SSO per connection, independent of how many users authenticate through it — a single enterprise customer with 50,000 employees costs the same one connection fee as a single enterprise customer with 50 employees. Auth0 and Clerk fold enterprise connections into their MAU/MRU-based tiers and overage rates, meaning a large enterprise customer's user count directly inflates the bill on those two platforms in a way it does not on WorkOS. For an AI SaaS selling primarily to a small number of very large enterprise customers, WorkOS's per-connection model can be dramatically cheaper than a per-user model; for a product with many small customers and comparatively few enterprise accounts, the per-user platforms may be more cost-effective since WorkOS's connection fee applies per customer relationship regardless of size.

Sensitivity

1. Your actual MRU-to-MAU ratio, for Clerk specifically. Determines whether Clerk's real cost sits near this article's worst-case $1,025 ceiling at 100,000 users or meaningfully below it; a product with many one-time signups and few returning users pays less than the ceiling implies.
2. Enterprise SSO connection count. The dominant cost driver on WorkOS specifically, and a meaningful one on Auth0's B2B Organizations line; irrelevant to a product with no enterprise customers needing SSO.
3. Whether Auth0's self-serve ceiling is crossed. Below roughly 50,000 MAU, Auth0's B2C pricing is predictable from its published tiers; above it, budgeting requires a direct sales quote.
4. Volume discount tiers on both Clerk and WorkOS. Clerk's per-MRU rate steps down from $0.02 to $0.018 at 100,000 and further at higher volumes; WorkOS's per-connection rate steps down across five tiers from $125 to $50.

Budgeting traps

• Equating Clerk's MRU figure directly with a competitor's MAU figure. They are not the same unit; this article's $1,025 Clerk figure at 100,000 users is a worst-case ceiling, not a confirmed real-world number, unless every active user genuinely returns.
• Citing an older ~$800-at-100k-MAU Clerk estimate. It does not follow from Clerk's current, confirmed published rate card ($25 base + $0.02/$0.018 per-MRU ladder).
• Assuming Auth0's B2C Professional tier scales predictably to 100,000 MAU. It likely does not; the self-serve path tops out well below that volume.
• Treating WorkOS's free 1M-MAU user management as the full cost of the platform for a B2B SaaS. The real cost, for any product actually needing enterprise SSO, is the per-connection fee, which is independent of MAU entirely and billed as a separate scenario, not a scaled-up version of the MAU table.

What to ask before you buy

Measure your product's actual retained-user ratio (returning users as a share of total active users) before budgeting Clerk's cost at any given MAU volume, since the worst-case ceiling in this article assumes every active user also counts as retained. If your product is approaching or exceeding roughly 50,000 MAU on Auth0's B2C line, start the Enterprise sales conversation early rather than assuming a predictable next-tier price exists to budget against.