I'd say its a decent deal. I just picked up my mini from the apple store for $880 US with tax, it was the stock i5 with 4gb, so you essentially get a "free" upgrade to 8GB and the i7 upgrade. However I would see if they would go $850 if not, and it IS in like new condition with box and all, then $890 is fair.
Retail you are looking at
Base $799
+ i7 $100
+ 8GB $35
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It would be $935 + tax new.