I built my first AR in the late 1990's; have built over 100 of them since then.
I generally don't consider assembling a lower and strapping on a pre-built upper to be "building," but that's ok - lots of guys go that route. I've only doneso once, which was a Dtech 243wssm upper - which can't be built by the average joe.
In general, you won't save money building a mil-spec carbine, you'll need about $100 in tools, and you'll only save about $50 on parts vs. a factory model. Alternatively, for factory models in the $1000-1200 range, you can build an equivalent or better AR for about $200 less, tools included.
I've built on a lot of DPMS and Bushmaster parts, several on Aero parts now. For what it's worth, the incremental differences between AR parts are minimal - stocks, forends, and grips will feel different for different shooters, and be more or less attractive to different shooters subjective eyes, but in terms of function, there's not $500 difference in functional performance between a $500 carbine and a $1000 rifle.
The important places to spend money in an AR are the trigger and barrel. The stock and grip need to fit well and be comfortable, but everything else is just subjective preference.
I personally only own one AR that's not a match grade custom barrel from Shilen or Black Hole Weaponry; a bushy M4 clone which I'm intentionally leaving as is. Mil-spec, $150 or less barrels can shoot well, but they can also be 2-3MOA barrels too. For a little more price, match barrels won't have that variability in potential, and they WILL shoot sub-MOA. I have a few barrels that will shoot in the 1/10th and 1/5-1/4MOA ballpark, even after a few thousand rounds.