I've seen assembled lowers going in the $230-250 range, but that includes a stock fire control group (trigger, hammer, springs and pins). Depending on what fire control group it has, it could be worth it. Rock River Arms 2-Stage run $120 by themselves. Gieselle and Timney are North of that.
I looked at Gunbroker and the cheapest one with an A2 stock was $259 with a seller putting a mess of them in at $309 (not a price point I would buy at with stock fire controls.)
Supply on lowers is low right now and demand is high. So what you saw on Gunbroker is pretty typical of what assembled lowers are going for right now, at least from what I've been seeing. I took a dozen lowers to the local gun show this past weekend with PSA MOE lower parts kits installed, and sold all of them the first day for $250 each not including a buffer tube and buttstock assembly. So at least around here right now, assembled lowers are running $250-300 if you can actually find them available.