The
Colt Trooper is close to the Ruger model you posted. It has a 16.1" barrel, M-LOK rail w/free float barrel. All the
Mil-Spec's that the M4 has. You could probably get on close to $600. You'd have to look around.
The thing with Colt is they hold their value. Like I said, the AR market is over saturated right now. It's a buyers market. Be patient and the deals are out there.
You might want to go over to m4carbine.net A lot of info over there. Do some searching around and reading. Some serious end users on that site with a lot of experience.
Good luck, NYH1.