Well, consider the GT300. The GT300's clocks are supposed to be the same or even lower than that of the GT200. The GT300's clocks will be nowhere near the clocks of the 5870.
And the highest OC on the 5870 (as we just saw), was 1300MHz - and it was strong enough to run 3DMark Vantage at extreme settings.
You see, you cannot do price per performance with high-end cards versus mid-range cards. Did you notice that nearly all the cards which were higher than the HD 5000 were mid-range cards? What you can compare the price is with other high-end cards (GTX 285, GTX 295 for now). Somebody who is interested in buying the 5870 is not going to consider the 4850 just because it excels in price-performance ratio. Also, note that the HD 5000 drivers are nowhere near final at the moment.
Anyways, my next reply to you will come in the evening, going out for now.