Don't think any of the SaaS billing providers today really suite SMBs in cost wise. Merchant account costs, transaction costs, billing provider costs could easily add up to around 10-20% of each transaction.
FastSpring's price is ALL IN. It includes the merchant account, payment gateway and subscription features, and no setup fees nor monthly charges; just the per-transaction costs as noodle commented.
Plus, sign-up and testing is totally free and still gets awesome customer support; you only start paying them anything when you start selling your service. This struck me as ideal for an SMB in its first year.
And this isn't about SaaSy. It's same with all services out there.