OK, lets be honest and not rewrite the history books.
A 4850, overclocked NEVER CAME CLOSE to a 4870 in performance, it didn't overclock very well and had a massively stunted bandwidth due to memory. A 4870, the top end card that wasn't even closely matched by its lower end 4850 brother, was £180 or so on launch.
This gen a 5850 which most certainly does match its big brother(5% difference both maxed out), which only has low stock clocks but isn't massively crippled like the 4850, was £200 on launch, I actually got mine for £193, about a week after launch.
The price is still £200 rrp, you can still find them for VERY close to this, you've been able to find them VERY close to this since launch. RETAILERS are marking them up, RETAILERS are making a killing on them because the consumer is willing(stupidly) to pay it.
SO last gen to get the best AMD card around cost £180, this time £200. What a massive difference.
The reason for the price difference is fairly simple, first GDDR5 costs several times what gddr3 does, that was half the reason for the low cost of the 4850. Also was a very mature 55nm process that had a cheap cost per wafer and massive yields for AMD.
40nm is a horrible process, wafers cost the best part of 50% more and on top of that yields are way down. Core costs are OVER 100% higher than last gen.
The fact that to get a similar level of performance, in terms of getting a top end card, has gone from £180-200, is brilliant. Because of the core cost increase to AMD, we could have seen the £300 5870 as 20-25% faster than a 5850, but a 5850 crippled so it could never get close, then you'd have a point.
As for looking at one game to determine performance, thats ridiculous, you could have gone with Metro 2033, probably the best looking game around right now, the 5870 is twice as fast as the 4890 in that, its showed the absolute best scaling in performance so far of any game.
Meaning, double the performance is certainly there. Not all games show it, hardly surprising and nothing new.