For 50 dollars less you can pick up at 5770 and those easily get 200mhs. It seems 2mhs/1$ is a sweetspot.
I wouldn't call 2 megahash per dollar the sweetspot. The 5830 @ 130 gives you 2.4 if not 2.5 and the sapphire 5850 has been said to be able to do 400 easily with no over volt that's 2.76 megahash per dollar. The super good deal on the 5870 a week or two ago ($189) can easily give you 2.3. The 5770 can be a good deal but not at 100 if you can get one at 70 or 80 maybe. That's 2.86 but keep in mind all the parts in your server dilute the megahash per dollar value. The only way I think a 5770 build could work is if you could manage to squeeze 8 of them on a mobo but I havent done enough research on that to be positive.
this is not true, non of my 5850 has been able to do 400mh/s on stock voltage overclocking. "easily" means most of them should be able to hit this speed, which is not true at all. this is only a few lucky good ones