For a first built, I'd recommend:
$99 Romanian parts kit from Century
$110 new chrome-lined barrel from Apex
$100-ish for a finished receiver, shipping, and FFL fee
$8 for rivets
redchevyman here on Gunco has good prices on the "922r compliance parts" you'll need to be legal, and greenmachine79 sells some really nice bolt cutter jaws for crushing the rivets. Or you can make your own; hcpookie came up with the idea originally, and there are DIY instructions here and on his web site. Or you can tap the trunnions and use screws; I did my first one that way.
If you want to build from a flat you'll need to build or buy a bending fixture, a welder to weld the rails in, and then either spot-harden the axis pin holes with a torch or go nuts and build a heat-treat oven. Then you'll probably want the usual Parkerized finish, which requires a heated tank, chemicals, etc. Lots of guys go this route - the fun is in the build, not the end product - but if you have limited resources, you can get a brand new receiver that's already bent, heat treated, Parkerized, and ready to rock for $100-ish. It will also have an ATF-registered serial number. This means you can legally sell the gun later, if you wish. If you build from a flat, it's perfectly legal to make and own without notifying any official authority, but selling it is a legal gray area.
If all you want is an AK, you can buy a new one for $350-$375, in a box with a warranty, and you'd be hard pressed to build one from pieces for that price.
**********DANGER WILL ROBINSON!*************
WARNING: Kalashnikov-based rifles are addictive!
You'll get one of the "precursor" Romanian or Bulgarian AKMs, and then you'll want a Krinkov, and a Galil, and a PSL, and then a Tabuk, and a couple more Bulgarians because they were on sale, and then the gun safe is full and your're storing rifles in plastic sweater boxes under the bed.
There's absolutely NOTHING wrong with that... but if you're not careful when you move into a bigger place to get enough room for all your AKs, you might strain your back while moving your gun safe. Back injuries can be serious! If you hurt your back, it can interfere with the enjoyment of collecting and shooting AKs. Let the movers handle the safe.