As for "the rules", I believe it had to be done by a person with the proper license. As far as cost, it wasn't real expensive--in hindsight--although I couldn't give you any dollar figures.
AR-15 drop-in sears were fairly cheap--$30? $50? No more than that. You had to do the whole registration bit, with the $200 transfer tax and all. I think the same sort of numbers for a Mac 10...
I know from nuthin' about the AK and MP $ figures, though.
Regards, Art