I realize it depends on the ASIC as well as GPU.. Just want an estimate for some calculations I'm doing.
Talking about Bitcoin mining, not an altcoin.
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The AntMiner S4 gets 2 TH/s and sells for about $1200. This is 1660 MH per second per dollar.
The AMD Radeon 5830 gets 307 MH/s and sells for about $200. It was released in 2010, so if we assume that graphics cards have doubled in power every 18 months, that suggests that the best Bitcoin mining graphics card on the market today gets 12.2 MH per second per dollar.
So they're on the order of a hundred to a thousand times less powerful per dollar.
This is a generalization.