Gartner Research VP, Neil McDonald & AlgoSec CTO, Prof. Avishai Wool, discuss the business-driven approach to automated security policy management, its benefits and how to align security policy management with business processes to address today's security challenges.
The cheap way to do it is put the wireless router on internet directly then put a firewall behind it and not allow traffic to pass from the routers IP's. This is not as secure but would work.. but you can pick up a router with DMZ appliance for fairly cheap and do it right...
Look at Sonicwall, ASA 5505 and Juniper all should do what you want.