saiga

So I know all about how to convert it and about the 922r compliance law, but I had questions on the front part of the rifle. What the difference between the Saiga and a normal AK. Besides the handguards/foregrip does anything else need to be changed for it to be a normal AK? Like the gas block...

I am currently in the market for a saiga 12 shotgun and have 500$ to spend. I would prefer not to spend that much and would like to buy a used one. Can someone please give me an estimate how much a used saiga 12 shotgun costs.