What's the best value flat top upper at the moment?
Just the receiver? or an assembled upper?
For just the receiver whoever has an Anderson on sale this weekend.
If you want something assembled, best value is going to depend on what you're doing with it,and what components you want. Absolutely cheapest firearm that will function is probably one of PSA's daily deals. Or what's on sale at Primary Arms. They have some Radical Firearms uppers under $200.
Looking at their site, Primary Arms will also give you a free upper and lower receiver if you buy a Giessele SSA Trigger. "Buy our trigger an you can have the gun to put it in for free."