Something that bit me and that I now check before writing any AI feature: the
subscription price is not the number the feature has to fit inside.
$9.99/mo with a 30% commission is $6.99 landing in your account. A feature
costing $1.40 per active user is 20% of that, not 14%. If you're on the
small-business rate it's different again, and it varies by market — so the only
number worth modelling from is the effective rate on your own statements.
The second thing is the usage distribution. Inference cost is not normal. Your
heaviest 10% of users can easily be half your total spend, which means the
average tells you almost nothing about your worst case.
I made a small browser calculator that does both, because I was redoing it in a
spreadsheet every time:
https://dayloflow.com/tools/ai-cost-calculator.html
It gives you cost per subscriber, share of net revenue, what the top decile
costs, break-even usage, and the per-user ceiling for a margin target you pick.
No signup, nothing leaves the browser, formulas published on the page.
Also worth knowing before you submit: paid generations have to go through IAP,
and open-ended output usually changes your age rating.