The cost changes so much from area to area and especially state to state so your best bet will be calling around to some shops and get quotes. I would try to stick to Mustang performance shops or at least a performance shop so that they will take better care to properly install them rather than just a repair shop or something that will just throw them up on the car and throw some quick welds on there.
Mine cost $70 per hour and it took close to 3 hours.
I was there to guide them through all of the Cobra specific process, mainly cutting the transmission cross member. Welding the passenger front side was tricky with the fuel lines. And as mentioned, make sure the work is done on a drive on lift.