I built two. First was a "complete rifle kit" minus stripped lower (which I got for $100. This is the part with the serial number, "The gun" legally, and the part you pay the gun tax on, so the smaller you get it the lower your tax) I got a Stag lower but there are many MilSpec makers (or rather, vendors- I guess there's only a couple of companies that actually "make" them, then put other names on them) and the kit was made up of Viet-Nam and later parts, I wanted the lighter 20" A1 barrel that came in the kit, didn't particularly care about twist so 12:1 was fine, for shooting milsurp 55gr that was just right anyway. They used an upper with A2 sights, not what I would have preferred but acceptable (and most people would prefer it). I'm not remembering or finding easily where I got the kit from but it was in the neighborhood of $300 several years ago, I'd suspect $500 these days. As long as you're using milspec it doesn't matter whether you buy it as a kit, you could assemble the parts you want from different sources, that's what I did with my second one- I wanted a lightweight 9:1 mid-length and a slick-side upper for that one. It cost more in the end but it was precisely what I wanted. Just an old-school carbine without all that tactical-Tommy crap hanging off it, built to carry, not for bench shooting.
You'll need a hundred bucks worth of tools, I personally don't count that as part of the cost of the rifle since I can re-use them or sell them and still have the rifles. You can borrow them or do a group-purchase if you have friends who want their own too.
Assembling the lower is an absolute piece of cake, many say you should just buy the upper already assembled but I'm glad I did my own. And of course you can swap uppers.
AR15.com is where you start, they have specific instructions with pictures, you really can do it. You may not save much $ on the first one but you get exactly what you want and you know how it's put together, to me that's worth something.