The prices quoted are high.
Any decent machine shop can handle the simple job of threading the end of a barrel.
The end of the barrel is turned to .500 inches and then threads are cut. The threads are 1/2-28, NEF.
The length of the threaded section, I don't know exactly, but I'm sure it's between half an inch and 9/16ths.
Provided you take apart the upper and provide them with just the barrel (The sight tower can PROBABLY stay on), I'd be surprised to hear of any reasonable machine shop (that's set up for small jobs and one-offs) that charged you more than 50 bucks to do it.