Firewall Security

HI

I am looking for a firewall for my network. That should be cheaper and best. Can any one suggest me how much brands are available in the market and how much surety of security in the mentioned firewall. I have a wired network of 30 systems with 3 Internet lines. 2 are leased lines and one is DSL. The problem in purchasing the firewall is that i want to use one firewall in all 3 lines. if it can possible please suggest me. Else tell me a good firewall for single line. but should be cheap and best

Thanks