Yeah, it's just the sweet spot for $ / credit right now. If you have the extra $ and PSU juice, then a 470 or even a 570 are VERY competitive for the insane credits they produce. But...going to be more $ up front and more electricity for sure, so ... has to be enough all the way around to go that route.
Here's
one on Ebay - I have one of these exact same models.
Also, if you live by a FRY's Electronics, then you can get one for $110 after rebate -
details here
A recertified PNY one from Newegg for $125
here
And lastly, I have one of these -
ASUS GTX 460 - $125 after rebate from Newegg.
Hope that helps a little, and happy hunting!