In the current environment, it has become difficult to build your own upper, even seeking sale prices, for cheaper than an assembled basic upper on sale, unless you have access to dealer discounts. The only way it really works is to wait for each piece to be on sale more than 15% off AND free shipping. Sales on complete uppers are just too easy. That’s before you even talk about tool costs. Or... you have to scrimp on some lower grade components and reduce the gross price, for example, getting blemished forged uppers instead of factory firsts, and a cheaper dust cover and FA kit, cheaper gas block and BCG, but then using the same barrel and handguard. The end result offers the same performance, but at a lower gross component cost, even before sales and free shipping.
Personally, I rarely have use for or interest in basic uppers, as most of the AR’s I build use will have $400 in the barrel and gas blocks alone. For a low cost, blasting AR, a guy really has to ask himself whether a $400 upper does anything more than a $400 complete rifle - and start asking themself if they really have a relevant use for a low cost carbine. Making noise sounds like a lot of fun until you have the money sunk, and then realize that blasting looses its novelty pretty quickly.