With an AK, you can get a parts kit for $100 or so (romanian in pretty good/real good condition). Then you need to get a reciever--you can buy one of those for about $70-80 (plus any ffl fees. I get mine at a local gunshow). Or you can make one yourself--that would be probably the most difficult part of an AK build. You'll need a bending jig and a press, then a welder to attach the rails. And if you make your own receiver for your own use, don't think it's legal to transfer to another owner.
Then you need to get compliance parts (the legal part is long and stupid, so I'm not getting into that discussion here...). Basically, you buy a new trigger, hammer, sear, etc that are made in the US. Figure maybe another $60 on that stuff.
Now, you've got to strip it down to get the rivets out of the trunnion. Supposedly, some folks can use a punch and a hammer, but I'm using a 12 press and a powersteering pump pulley puller to get mine stripped down.
Some folks drill out the old rivets then tap and screw the rifle together--I'm not sure how they can get all the old rivet bits out without removing the barrel, but supposedly its possible.
Anyhow, it's a blast and rewarding, but you should research it thoroughly before deciding to go that route. If you're making one or two, it'll probably not be cost effective. If you're like me, you'll want a Yugo model, a Romanian model, an AK-74 and a Polish Tantal. Not to mention the best of the AK kits (IMO) an AMD-63 from Hungary. If you're doing it to save a few bucks to get an AK, I'm not sure it's the way to go.