Editorial note. ElevenLabs figures are from elevenlabs.io/pricing; Zapier figures from zapier.com/pricing. Both as of September 2026; neither company is affiliated with Sonic AI. ElevenLabs' overage handling changed in the past year (a legacy per-1,000-credit rate versus the current usage-based system), so the overage discussion describes the shape — confirm rates on their page.
ElevenLabs — the voice AI company — is the reference example of the model almost every API-first product ends up with: a ladder of monthly credit allotments, a price per rung, usage that draws the allotment down, and unused credits that carry forward for a while. What separates products on this model is what happens at the edges — when you use less than the tier, and when you use more. ElevenLabs answers the first with rollover; Zapier, further down, answers the second with a multiplier and a cap. A billing system that sells prepaid tiers needs both answers to be configuration.
What ElevenLabs charges
Credits per month:
| Plan | Price | Credits / month | Rollover |
|---|---|---|---|
| Free | $0 | 10,000 | None |
| Starter | $6 | 30,000 | Up to 2 months, balance ≤ 3× quota |
| Creator | $22 | 121,000 | Same |
| Pro | $99 | 600,000 | Same |
| Scale | $299 | 1,800,000 | Same |
| Business | $990 | 6,000,000 | Same |
Credits burn per character (or per minute), and the page also lists straight API rates — e.g. text-to-speech at $0.10 per 1,000 characters on flagship models, $0.05 on the fast ones — for usage outside a plan.
The model underneath
Both are a prepaid usage slab: a committed monthly price buys N units; the ladder is a list of slabs on one product; usage draws the slab down; the effective unit price falls as you climb. Not graduated — the millionth unit inside a tier costs the same as the first; you choose a slab.
The two edge rules:
| Edge | ElevenLabs | Zapier |
|---|---|---|
| Under the quota | Unused credits carry forward for two months, balance capped at 3× | Lapse at month end |
| Over the quota | Overage per unit at the plan's rate | Overage at base rate × multiplier, where base = plan price ÷ included tasks; multiplier depends on billing term; hard cap at 3× |
What a month looks like
Pro ($99, 600,000 credits), 420,000 used in August and 710,000 in September:
| Month | Opening | + Quota | − Used | Closing | Charge |
|---|---|---|---|---|---|
| Aug | 0 | 600,000 | 420,000 | 180,000 | $99 |
| Sep | 180,000 | 600,000 | 710,000 | 70,000 | $99 — no overage |
Without rollover, September would have carried overage on 110,000 credits. One rule changes the bill; it has to be explicit configuration.
Who else sells credit tiers — Zapier, and the overage-multiplier version
ElevenLabs answers the under edge with rollover. Zapier answers the over edge: you choose a monthly task allotment from a ladder, and tasks beyond it are billed one by one at 1.25× (annual) or 2.5× (monthly) your effective per-task rate, with a hard stop at 3× the tier. Same prepaid slab, opposite emphasis. Midjourney, Notion AI, Runway and most "credits" products sit somewhere between the two, and every one of them needs both edge rules to be configuration rather than a support policy.
Zapier — tasks per month:
| Plan | Entry tier | Annual | Monthly | Tier range |
|---|---|---|---|---|
| Free | 100 tasks | $0 | $0 | — |
| Professional | 750 tasks | $19.99 / mo | $29.99 / mo | 750 → 2,000,000 |
| Team | 2,000 tasks | $69 / mo | $103.50 / mo | 2,000 → 2,000,000 |
| Enterprise | Custom | Custom | — | Annual task limits |
Overage, from Zapier's page: with pay-per-task on, extra tasks cost 1.25× the base rate on annual and 2.5× on monthly subscriptions, and "usage stops once it reaches 3× your task subscription."
Zapier — Professional 750-task tier, annual ($19.99), 1,100 tasks run:
| Line | Qty | Rate | Amount |
|---|---|---|---|
| Professional — 750 tasks, September (in advance) | 1 | $19.99 | $19.99 |
| Tasks beyond allotment (in arrears) | 350 | $19.99 ÷ 750 × 1.25 = $0.0333 | $11.66 |
| September | $31.65 |
On monthly billing the overage rate is $29.99 ÷ 750 × 2.5 ≈ $0.10 per task — $35 for the same 350 tasks, more than the plan itself. That nudge up the ladder only works if the billing system derives the overage rate from the plan the customer is actually on.
Where prepaid tiers go wrong
- The balance is state. A rolling balance per customer per product, carried across periods and capped, cannot live in a spreadsheet once there are plan changes mid-month.
- Overage rate hard-coded. "$0.03 per task" breaks the moment a customer moves up a tier; the rate belongs on the slab.
- Term changes the multiplier. The same product has two overage rates depending on annual versus monthly — a per-schedule attribute, not per product.
- Caps are a billing rule. Usage the product refused to run must not appear as billable events.
- Mid-month upgrades. Creator → Pro on the 15th: does the new quota prorate, does the old balance carry, is the price difference charged for the remaining days? Each shows up on the invoice.
- High-volume, replayable feeds. Credits are drawn per request; a re-sent feed must not draw twice.
Billing this shape on Sonic AI
Both ladders are one product with a usage slab list price on Sonic: each slab holds its committed price, its included units per commitment period, and its own overage rate per additional unit — so Zapier's 1.25× (or 2.5×) is a number on the slab the customer selected, not a formula hidden somewhere. The schedule holds the selected slab and the carried-forward units from the previous period, so ElevenLabs-style rollover is computed by the schedule rather than reconstructed by hand. The slab bills in advance; overage bills in arrears from the same meter onto the next invoice. A tier change is a schedule phase boundary prorated by calendar units. Events land through a usage template or a Postgres sync with idempotency on each event.
See usage & seats, usage-based billing, explained, and upgrade & downgrade proration.
If you price like ElevenLabs
- Do unused units roll forward, for how long, with what cap?
- Is the overage rate on the slab, and does it differ by billing term?
- Do you cap usage, and does the billing feed respect it?
- On a mid-period tier change, what happens to balance and price difference?
Bring your tier ladder to a demo — we will configure the slabs, the rollover and the overage rates on the call.
Similar pricing models
Prepaid tiers of credits with an overage rate are the model behind Zapier (above), Midjourney, Runway and Notion AI; the same slab attached to a seat is Cursor; the same slab at the scale of a year is Snowflake capacity. With no allotment at all — pure pay-per-unit — you are at OpenAI.
Related terms: overage, usage-based billing, schedule phase, in advance vs in arrears.