What you aren't taking into account is that both of those cards were considerably less than the 7970
I have taken it into account, hence the £700.
The 590/6990/5970 and so on (and the GTX 295) cost almost double their single counterpart with a very small discount.
5870 was what? £300 when prices settled? 5970 was about £500.
How much was the 6970 at release? £300? (don't remember now).
6990 what? £550ish?
And so on. Single slot dual GPU cards are always hideously expensive due to what they are designed for. They are designed, quite simply, for a company to say "We have the fastest single PCIE slot card".
Thus, they're not priced very keenly as they are more a marketing ploy than something that should be taken 100% seriously. Anything that relies heavily on driver and game support will never be mainstream.
Recently when faced with the proposition of being able to splash out on a card with money (sort of) being no object I looked very closely at the 590. Sadly the price per performance was absolutely ludicrous. I could almost have bought myself two full fat 580s.
And the same rings true for the 6990. Infact, IIRC, it would have been cheaper to buy two full fat 6970s.
My £700 guesstimate relies heavily on Nvidia having nothing to show by the time it arrives. Otherwise it will be cheaper to suit.