Price:  Price of a cheap box, either free to 500$

OS:  FreeBSD firewall.

Works pretty darn well, and won't bust your wallet.  Others here may have better ideas, but that's my 'firewall on a shoe string'. 

Seconded, the m0n0wall and PFsense family usually provided everything you need. Just be wary if you're running this in a heavy usage environment as it's performance can start to suffer at 20Mb+ depending on ruleset optimisation and utilised features.

Can't think of anything better for small environments though.

Agree. Smaller netscreens and pix's are also good options. Provided you can find them for a good price. Also be aware, you may or may not be able to get software updates for a second hand device, vendor policies differ.