£850 for a 3 thrashed graphics cards and half a PC ? I reckon you'd be lucky to get half of that. Do you realize that you could buy 18 Block Erupters for that which will give you ~6 GH/s ?
With respect, you can't mine anything else but BTC with USB erupters - and you definitely can't play games with them. In 2 -3 months those erupters will be worthless due to diff. GPU's are dead for BTC minig - but great for everything else. And they have resale value.
Give the guy a break eh?
Peace.