A friend of mine bought a RRA Entry Tactical for around 1100 new, and sold it for 900 around a yr later. It seems like that is about average range depending on options and condition.
I saw one on Arms list for $750 about 2 weeks ago. Guy was looking for a down payment on a Harley. Thought about picking it up because it was a great price, but couldn't justify buying it due to the fact that I purchased a new Beretta PX4 subcompact 2 days prior...
I sold a couple w/in the last year that were somewhat loaded up for <$900 (one had 0 rounds, the other about 4-500.) To me, pricing really depended on the options (free-float handgaurd, decent stock, sights, etc.) I would second the idea above to build the rifle on the RRA site and decide what discount you feel is justified (normally at least $2-300 from list since they are not that rare of a firearm and can normally be found at or below list from stocking dealers.)
If you are not set on a RRA by name, it is worth looking at a build. Depending on what you have or can borrow for tools, you can normally save yourself some $$ while getting your AR exactly how you want it. Otherwise, you end up paying almost the same money, but you still get just what you want and have tools on hand to either sell or use for your next build ... either way it's a win.
my uncle bought a RRA Operator new at a gun show loaded with extras (well, standards for the rra operator), including chrome bolt, upgraded butt stock, and freefloat rail, for only $800. i shot it (m855 and m192) and it is of the highest quality. shot smoother than my colt 6920 actually, and was quiter due to the upgraded RR compensator i think, or maybe because it was a midlength, but i would buy it instead of colt if i was shopping due to the value.
colt is probably a pound or so lighter, and has 1/7 instead of 1/9, but those arent big differences imo for a plinker. colt 6920 cost 1300 and comes with absolutely no frills.
I wouldn't buy a used AR right now. The retail prices have come down some, but the used prices are staying up because they paid more when they bought it.
I had this situation a few years ago when I bought my Tundra. I bought it new for what people were asking for used ones.