I wouldn't put rock river in a Cadillac section at all. They are midrange IMHO, based on a collection of reviews and their specs.
Best bang for your buck (same specs as the above manufacturers, but not grouped with them due to things unrelated to the product itself):
Spikes (hard to find in stock. Great reputation, they are last years PSA. Newer company just getting established, prices are great)
Palmetto State Armory (this is my upper. They are a smaller company SWAMPED with orders, it may take a while to get parts. They are the new Spikes. High quality, but still new on the big scene and struggling to keep up with demand and It shows if you try to actually get ahold of someone there for CS.)
Bargain: (this will probably piss some people off)
Most "Bubba guns"
Other manufacturers such as S&W sell a cheap, bargain sport model without the high quality specs, all the way up to very decent rifles.
If you have $1500, there is NO reason not to go with a BCM or Daniel Defense rifle. The other things you have to decide are what type gas system, FSP or Low-Profile gas block with an extended rail, tubular or quad rail (recommend tubular), optics included in the price? If so, scope or red dot sight, any additional furniture. For $1500 you should be able to build a complete rifle of the highest quality parts with a good optic.
"They who can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." -Benjamin Franklin