I don't think you want to go too "cheap" when it comes to your billing software. The price WHMCS charges for their application is VERY reasonable for what you get. Plus there's a huge community of people in their forums should you need help, and lots of people recommending the software.

I don't use it myself, as my business is different. But from the amount of times I've heard recommendations for it, AND from my personal dealings with Matt (owner of WHMCS), I would definitely recommend WHMCS.

Rob.