Not only that PSU is expensive, it's garbage too. It's advertised as 700W unit, however, having looked at specifications, I can tell it's no more than 500W. It has low power on 12V, and that's what you mostly need. Furthermore, it has no safety certifications, so if it blows up - it's likely to take whole your PC with it. That CX 430 is cheaper, comes from reputable brand, which means only quality materials were used to manufacture it, has only 100W difference on 12V rail. It's more than enough to run HD 6770, hell, it could even run HD 6950!
Check this:
http://www.hardwaresecrets.com/article/Corsair-CX430-V2-Power-Supply-Review/1284/8
It could withstand 120W more than what it's advertised to be! And it didn't blow up - it safely shut down like it should be.
Check what no name brands power supplies can do:
http://www.tomshardware.com/reviews/low-cost-psu-pc-power-supply,2862-3.html
Hope it's become clearer on how important good power supply is for your gaming graphics card.