Palmetto State Armory. Look at both built and ready to build uppers and lowers. Many options. Likely the cheapest and you can upgrade anywhere you prefer. i.e. trigger, barrel, furniture, etc.
This, but one problem going super cheap is you're not only giving up accuracy, you're giving up reliability too. FWIW, Noveske's .300BLK barrels are both accurate and they cycle supers and subs, can or not with an 8.5"bbl. I have two of 'em plus a 10.5 (but haven't fired it, it's actually a numbered collectors rifle I got for the ex wife but I'd suspect it does fine). Then a 16" AAC. I can't really speak for the others, Noveske is just my go-to for SBR barrels, period --I find a good mfg. and just stick with 'em. They just work and I don't like fucking around with adj. gas blocks and shit and usually they're at least 1MOA, maybe better, depending.
You won't find a blaster cheaper than PSA, I don't know how you can make one that cheap, but I've learned it costs less to do it right the first time, especially if you're gonna depend on it at all. If I were you, I'd build it so at least you can pick a barrel and a BCG that you can depend on if nothing else.
I'm the guy that Form 1'd a Mega lower so I built a "cheap" SBR using that and some other decent parts I had laying around:
I guess it's not really that cheap but I wasn't able to build one shorter that's more reliable. It's around 21" I think, not too heavy but it is heavier than it looks IMO; that stock ain't that light and neither is the carbon fiber tube despite being carbon fiber (it's thick as fuck) but I really like the triangular shape on this one (I'd probably use these tubes more often if they didn't require special sights or top rail to make level). 5" TBAC Ultra doesn't add all that much to it either. The irons are fast and cheap and all that's really necessary for this. Now you could go cheaper with a different stock and handguard and a Brownells/Geissele trigger (the best trigger out there for the price when they're 35% off) but I had these laying around. The BCG is milspec, nothing fancy.
This short a barrel is much more conducive to the .300BLK than 5.56 too. Better performance all around. I always liked the LaFrance M16K for a shorty, this is pretty close and in a better caliber.