Do you have a specific caliber in mind? https://www.theoutdoorstrader.com/forums/handguns.72/ https://www.ar15.com/forums/Equipment-Exchange/AR15-Complete-Pistols/174/
Don't forget to check GunBroker.com either. Open a free account if you do not already have one. Their advanced search can be useful on both current and completed items to help you gauge current pricing on particular models of AR pistols.