With a limit of 50MB per person they could probably operate this with a residential BB connection of, say, 30GB, and wifi modem in each restaurant.
I'm curious what wifi hardware is enforcing the 50Mb limit.
A client of mine is having their free wifi abused really badly, so looking for a way to easily implement this. Hence my questions about who is running the infrastructure.
Give Tomizone a go, if you grab a Linksys WRT54GL and install the Tomizone Software on this you can do it with ease.
If you need more access points for your client, then the free and opensource software PFSence has this built in, I have installed this a few times too. All you need to do is have a PC with 3 network cards (1 for WAN, 1 for Local Network and 1 for the Wireless AP's) - From here after a bit of config any user connecting to the network over the Wireless AP network will be greeted with a page either requesting a login, or a disclaimer page.
The advantage of using PFSence with this, is you don't need to purchase additional hardware, all you need is a old computer and you are set. If you want a embedded solution then it's possibly worthwhile grabbing a Soekris to get the job done, these are embedded computers with standard x86 processors. Also you can hook this setup via a switch so people can have ethernet access too.