Does anyone have a chart to show how much price per terahash has been historically, or what it is on average?
The most efficient GPU (in terms of Mhash/s/$) is the ATI HD 5830 which scores 3.28 (whereas a 6990 scores a 1.23).
The most efficient FPGA is the BFL single which scores a Mhash/s/$ of 1.64 (but provides better profit than a 5830 after considering the cost of electricity).
The most efficient ASIC is the Avalon ASIC which scores a Mhash/s/$ of 52.34. Followed behind that is the BFL Mini Rig SC at 50.16. The BFL rig might yield better than the Avalon ASIC as it is more efficient on a Mhash/s/J (power consumption), but electricity consumption is nowhere near the most important factor these days (... delivery dates are).
These numbers are from the Mining Hardware comparison article:
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http://en.bitcoin.it/wiki/Mining_hardware_comparison#FPGA[Edit, Oops. fixed, added the K, thanks D&T.]
So a price of hardware per Thash/s would be the inverse I suppose,
GPU: $305K per Thash/s
FPGA: $609K per Thash/s
ASIC: $19K per Thash/s
