It seems a fair price, does it come with a warranty? You can get new machines custom built with i7 2700K processors and a range of graphics cards from £520 ish. Take a look at this link as and example - I have never used them so have no idea of their quality or reputation.
I have priced the components to around £650 and I think who ever told you that to build it brand new would cost £1500 is telling porkie pies. If you are paying £700 then that is a fair price. Case is only around £60, had one of these once,not a great case but not a bad case either. You might want to ask what power supply they are using.
But all in all not a bad deal.
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