It's pretty much a question of what someone wants. It's a great project if you like tinkering and making stuff. And if you don't mind buying more parts to maintain 922r compliance.
If one is looking at absolute rock-bottom price, buy a SKS, since they are still cheap, just not as dirt cheap as before. As far as ammo goes, 5.45mm ammo is scarcer, and offers less choice than 7.62mm or 5.56mm NATO.
Oddly, prices on "generic" AR15s are dropping, due to so many manufacturers and OEM parts suppliers are getting into the act. Everybody wants to get into the act of selling "America's rifle". No problems with 922r compliance. You just have to buy a serial-numbered lower receiver.
M4 clones can range in price from $700 to $2400, and yet anything in that range can be a good or bad deal, depending on what you want. I've had extremely good luck with DPMS on prices and qualities, making them a good buy among second-tier manufacturers. I stumble across ads from a lot of dealers offering $100 or more off all the time. I like DPMS because it tunes up real well, if you know the tricks.
I still feel a M4 clone offers the lowest overall cost-of-ownership, when you throw in ammo, spares, and common sources of parts. It is also ridiculously easy to learn to shoot it effectively. And it is now so common, a lot of people out there know a lot about it, so you spend less time building, and complying, and more time shooting. YMMV
5.45 scare? I can get still get surplus ammo for less than $300 for 2160 rds. At that price I can afford to go shooting regularly.