My local gun ranges won't allow steel ammo (sparks, potential fire hazard) so I deal only with brass-cased and no steel core (no inexpensive Russian ammo ).
1000rnds of factory .223 for the AR runs about $300 delivered. 1000rnds of surplus Yugo 7.62x39 for the AK runs about $230 delivered. Russian ammo will be less expensive in both cases.
I built an AR-15 first. As others have pointed out, it's pretty simple. I bought a stripped lower and some California-specific parts at a local shop, and everything else was purchased online and shipped directly to my house. I opted for a complete upper because I didn't want to get into assembling that. The lower assembly was pretty simple with a parts kit and online instructions. I have a brass hammer and punch set and some electrical tape, and it took less than an hour. I'm guessing that it could have been done with a rock if you don't have fancy tools like a hammer and punch set.
The AR is fun and accurate. I installed a 4X ACOG on it, and I can hit an 8-inch steel plate at 400yrds pretty reliably. I can't wait for the zombie apocalypse!
The AK is still under construction. I bought a Saiga in 7.62x39, parts to convert it to an AK-47 and American-made wood stock set. I've disassembled the Saiga, drilled out all the parts I don't need, and put all the metal through a parts cleaner to get it ready for paint (Duracoat). After paint, I'll put it all back together in its new form.
I've sanded down the stock set, and I'm in the middle of staining it deep red/brown. I'll follow that with many coats of polyurethane and light sanding.
The AK conversion doesn't need to be so involved, but I choose to do it. In any case, building or assembling an AK is going to be more involved than assembling an AR. In the end, I don't expect it to be as accurate a shooter as the AR but I'm sure it'll be fun!
Keep in mind that building/assembling these guns is somewhat like building/assembling computers. I do it because I like making choices on what I want, and I do it because it's fun.