What's the cheapest solution to charge my SaaS users?

I run a webapp, I’m not going to say it’s name so as to avoid self promotion.

It’s getting to the point where I need to cover some of the costs I’m incurring for Slicehost server space and Amazon S3. I’ve looked into Paypal, Chargify, Braintree, and most of the payment/merchant providers on this list: http://wiki.github.com/Shopify/active_merchant/gatewayfeaturematrix

I’m pretty sure I may get like 2–5 people paying if add additional features which you can pay for. But that’s only going to be $10–25 a month in charges.

All to say, while I’m still proving this website’s viability, how can I make it viable for myself–as I don’t have much money to throw at it.

EDIT: By cheapest I mean like $20 a month. I know I’ll get what I pay for to some extent. But think in that range (if you know a lot about this kind of thing).