Solid idea, after having built N billing portals in the past, I really like the idea of being able to drop in a billing portal.
That said -- raise your prices. You might try something that's more explicitly tied to revenue. But if not, I would try the 3 tiers at 39/99/199 (or 49/199/299). Maybe make a 0-10 customer tier that's lower cost. The bigger customers will demand more features to justify the price point, but many of those features will likely prove relatively easy to implement (SSO, 2FA, let them pay you via PO).
That said -- raise your prices. You might try something that's more explicitly tied to revenue. But if not, I would try the 3 tiers at 39/99/199 (or 49/199/299). Maybe make a 0-10 customer tier that's lower cost. The bigger customers will demand more features to justify the price point, but many of those features will likely prove relatively easy to implement (SSO, 2FA, let them pay you via PO).