are you in the us?
Edit: Here are a few i found just quickly from http://www.newegg.com
which are rated at 500W max and have 70% and 80% efficiency...
PSU 1 for $49.99
PSU 2 for $69.99
They are both SLI capable, with 2 12v rails, and two PCI-E connectors. PSU 1 has 70% efficiency at full load, and PSU 2 has 80% efficiency at full load. Both also have a 20+4 connector for the motherboard - which means it can be changed to suit a 20-Pin motherboard power connector or a 24-Pin connector. Hope it helps.