Turn a Mac Mini into a public WiFi hotspot
You could do it with LinSpot. They take a 15% cut of for-pay access. You can install their software on your Mac. http://www.linspot.com/

As for doing it yourself, it's a bit trickier. You'd have to run your own DNS server, so when a user connects to your network, the first request would be redirected to your splash page. Then all subsequent DNS requests would pass through your server to a real DNS server. That type of policy may be possible with OSX Server or iDNS server, but I'm not 100% sure on how it's done. You will need server administration software, and you will need capture the DNS to create your own Captive Portal (as it's called).