Presently the GX2 is kinda cheap, can be had for 70-80$. It does around 11kPPD but outputs an excessive amount of heat. I have seen some recent numbers for GT430, I think Zodac from OCN had some results, and some averages too, but I seen it can go as high as 8k ppd. GTS450 also does a lot even with new units. some results here
[MPC] GT 430 - Folding Results - PPD/Power Usage
GT430 can be found also in pciex16 and simple pci guise at around 55$ on tigerdirect. Most of them are single slot and some are low profile too.