Threading a barrel is usually between $40 and $75, and the nice flash hiders start at about $45, though A2 style birdcage hiders are as cheap as $8. Muzzle brakes start at about $45 and run up to about $200 for some more exclusive labels.
The best reason to do this is it allows you to swap out at will, trying differnt things until you get what you want. It also opens up the door to a suppressor if you care to pony up for a stamp and order one. Shooting a suppressed rifle is a blast.