This looks like a pretty solid deal when you look at megahash per entire server cost. I don't know the exact numbers though because I'm not sure what is considered a reasonable speed to get out of a 5870.
I'm getting around 440MH/s out of mine. I can't remember where I saw it, but there was a chart comparing MH/s, cost, and power consumption and the 5870 is THE card to get for MH/s vs. Power used.