NetBilling may be perfect.

Just check out (unless Mitch wants to post the amount)

1. Monthly minimum or fees
2. Per item cost
3. Gateway costs (and per items extra)
4. Chargeback fees
5. Setup fees

We worked with a few websites that NetBilling was the best choice, others that CCbill or Epoche, and some that went for a direct billing solution (your own merchant account).

It all depends on volume, risk, and products.

But at least they have a module for X-Cart.
You can do the same with just about any other provider and we can help out if you need.