WHMCS is a good choice..
It allows you to automate billing tasks, create customized invoices, offer flexible promotions, and a myriad of other billing related tasks.

Don't just focus on the cheapness, you should also think of the reliability & security etc.

I agree with the above but also IMO the free or cheap billing / support software actually creates extra work where as WHMCS makes everything more stream lined.

So if your time means anything to you a good paid billing / support software will save you heaps of money just in time saved.

WHMCS is better it get for you full control

You can get WHMCS free with some hosting companies like hostgator and site5 with their reseller plans.