Tons of stuff out there on this topic, but a basic run-down of what to avoid would be
1. The WASR(for me anyway, some people love them)
2. chrome lined barrel(a must if your shooting corrosive stuff)
3. chrome lined bolt carrier(not really neccesary but they sure do look nice)
4. rust(a little around the screws might be had but anything excessive should be passed on)
5. country of origin(saiga converts, bulgarian, hungarian, chinese, and egypthian are usually regarded as the better manufacters in the ak world)
6. Fit and finish(pop the dust cover off and give it a look over, shake it around a bit and see if anythings excessivly loose, make sure mags fit nicely[not too tight or loose], and just make sure the parkerization or other finish isnt just beat to hell)
Prices range vastly from $350-400 for the WASR to $1500 for the SA-M7, so pricing can be difficult but a polish underfolder from CAI should be around $500 with a little wiggle room. at Cabelas ill guess they charging around $550-$600