The cheapest way is to shop continuously, know current prices, and jump an good or better quality parts when on sale. If you assemble you own upper hopefully you can borrow the few special tools you will need, otherwise probably best to look for complete uppers, a lower, lower parts kit and butt stock parts "on sale".
As long as you avoid Hesse/Vulcan/Blackthorne/whatever-they-call-themselves you should be fine.
For example, Primary arms had Aero Precision forged lowers on sale this morning for $69.95, but they are sold out already, they are trying to get more to sell at that price.
Get on the "Email specials" lists from all the major AR component suppliers -- sometimes "marketing spam" will have exactly what you want but you have to move fast! I placed a web order for the AP lower at 8:30AM this morning after getting their Email, and drove done to PA's storefront to pick it up and do the 4473 (avoids shipping and FFL transfer fees, so a local source for the lower is usually best) at 11AM and they had sold the last one on-line before I'd left.