670 Ghs will generate 0.1752 BTC (103.3$ @ 589.8$/BTC) in one week at current difficulty.
Why will someone pay 140$ to get 103$?
Because some people really don't know a thing about mining, so they'll buy a few GH/s and see if they work. When truthfully, you'd need at least 1,000 of 'em (Or, one TH/s) to actually get something going.