Probably not true for everyone, but I would never implement this if it took a percentage of revenue. I made that mistake once. This is a very basic SaaS feature you can build out once and rarely think about again, or just grab a prebuilt starter SaaS setup like the ones I know of for Rails or Laravel and start with that. So I’d be on the fence about a monthly fee to begin with. But a percentage of revenue? Hell no!

Edit: plus, since you’re not driving new revenue, and replacing you is a one-time-fee (maintenance for this feature is minimal), all you’re doing with pricing based on percentage of revenue is pushing your largest and most valuable customers to get rid of you as fast as possible.