To be honest there is a lot of speculation about these new cards and the prices they might come in at, not in the least bit helped by the Nvidia titan, which as we all know is way out there in terms of price.
Some people will keep telling us all that they will be 5/6/700 pounds, others will tell you that they will be 3/4/500 pounds. My self I think the 770 will come in around the price of the currant 670, about £350 and the 780 about £450.
You can get a GTX 670 for £289, which would translate to $359 before tax (that is the actual price in the States too). It's more than likely that GTX 770 (that's supposed to be an overclocked GTX 680 GPU) will come down under that price point, although it's all speculation at this point, as how I explained it earlier, there's no such thing as a UK RRP and prices will be adjusted based on the US market prices and a number of other factors, which is why it's more sensible to discuss those rather than what the UK launch price will be.
From what we know so far, it's a given that the prices of GTX 670/680 equivalents (so GTX 760 and GTX 770 accordingly) will be favourable compared to the current prices, and hopefully they will be priced that way.
For comparison, the launch prices:
GTX 570 - $349
GTX 580 - $499
GTX 670 - $399
GTX 680 - $499
If the GTX 780 is actually a Titan LE, and all sources so far point to that, there will be quite a price gap between the GTX 770 and GTX 780, possibly as large as $200.
So say GTX 770 (slightly faster than the current GTX 680) comes down at $399, we might expect the price to be around £319 after VAT, which is quite reasonable considering the current generation's prices, but disappointing when you take into account how long we've waited and the small performance increases for money.
That would put the GTX 780 at $599, which is around £469 with the current exchange rates and inclusive of VAT.