I was quoted more at a shop near me to have shocks/struts/spring/rear panhard bar installed on my '12 GT. I did it myself though. It's really quite easy. If you're going through the trouble of putting in new shocks, you may want to consider doing springs while you're in there.
Not being a pro and being the first time I worked on any mustang, replacing the rear shocks/spring+panhard bar install took me an hour. I was also taking my time. The fronts probably took me 45 mins per side--again, taking my time, never having worked on a mustang or Ford before. I would say that anything <$300 is probably where you want to be if the shop is being close to honest with the hours they're charging. But it will all depend on the shop--I take my cars to a very expensive indy shop because I think they're worth the price. They do a lot of track prep for BMWs, Ferrari's, Porsche, and they also do a lot of the overflow installs for American Muscle. If I can't do it myself, I want to take it to a shop where I know I won't have to worry about some guy half assing the work, rushing, or just trying to get by. That's also why I don't let them do easy things for me either---they cost too much for that!