Highest I've seen is $3,850 on Gunbroker.Perhaps the highest I've seen them go for: $3,325.
NEW - HK MR556 NOT noveske, colt, lwrc : Semi Auto Rifles at GunBroker.com
Starting Bid was $3,400
Buyout was $3,850
Date: Nov 4 2016 (before election)
I wonder if the seller was a FFL, in order to profit from the firearm trading...
Sometimes I think the bid war in GB can become a personal pride issue. The rifle you noted was $2,900 for a few days. It went up to $3,300 range in the last 15 minutes. Sometimes people just want to screw with the highest bidder by jacking up the price... until they become the highest bid and no one else bid anymore...
That's how I ended up with my ACR.