How did they convince you to buy those cards? Not trying to be a downer, but they are overpriced for what you get.
For mining? 6990 is one of the best for price to performance. The difficult part is finding one, as it is sold out almost everywhere.
They are actually the opposite of this statement. A $700 dollar card (usually more) that puts out 800MHash/sec (often less) is $1.14 / MHash. A 5830 is about $2.85 / MHash.
Even factoring in stuffing 8 GPUs onto a single motherboard usually doesn't even out the cost ratio.