So these numbers are completely based on intuition and architecture, but I'd expect somewhere in the 10-15 MH range for a 270. I have no idea what the power usage would be. The biggest problem is you also have to get the other components for a rig, which is ~400 bucks on the cheap end of things. Since ETH is kind of unknown in terms of how long mining will happen, it's hard to say if it's worth buying old hardware just to mine ETH.
It's sorta like the problem with buying S4 when S7 is available. Sure, it's a faster payback period, but will it be profitable long enough to pay itself back. That's the gamble.
edit: Ouch, just realized you said R7 240 not 270. Yeah, those are only a couple MH each. The R7 250 is like 3 MH on ETH.