Why are you comparing it to the HD 4870? The HD 4980 was the top AMD single GPU card at the time of launch for HD 5870.
If you want to play that game:
HD 5870 launched at $380, versus HD 7970 $550 == 45% increase
Performance HD 7970 is about 60% faster.
The HD 7970 doesn't have more performance per dollar? It's a HALO card. I didn't know top end cards are suppose to have that as a requirement. Well shit, no top tier card had great Perf

ollar since the GT 8800. It's all been downhill since then for top tier cards.
The HD 6970 wasn't a halo card, it didn't launch first, and it was priced to compete with the market. Same with the HD 4980. HD 5870/HD 7970 launched first and were priced to compete with the market.
But, If IF IFFFFFF, I get it, and if the world ended tomorrow, your argument is still moot.