The 720q build I linked to previously, is around $260, if you buy most of the stuff used, on ebay.
It'd manage 2.5gbit, with SQM, if you get adequate NICs.
If you don't need more than 1gbit, and skip SQM, there are several cheaper suggestions at the buttom and top of Tips for getting cheap used x86-based firewall with full Gbit NAT (a PC Engines APU) if you are in the US