Setting Up An Open Source Billed Hot Spot

Hey Guys. I’m wondering if any of you can help me on this. I want to set up a hotspot where I will charge the users.

I want an open source solution to point users to some kind of splash page where users will log on from a prepaid card to use it. I have done some searching and gotten some of them but they involve changing the firm ware on a linksys box. I dont think the access points I will be using are compatible…can anyone help me out?