I took my 69 Mach 1 seats, new buns and new covers to a small local shop "on the wrong side of the tracks" so to speak. They did a great job! I think I paid $150 or $175 for each of the front seats. I also took them a can of black spray paint and had them paint the seat frames before re-upholstering them.
One thing I've noticed, both with the Mach 1 seats as well as my Pantera seats (same shop and also TMI upholstery) was that the new seat buns are much taller than the originals. I found that in both cars I sit about 1-2" higher than I used to in the old seats. Maybe it's just because the old foam seat buns were broken down, but I used to sit "IN" the seat and the side bolsters were higher than the center section and provided a modicum of side support. Now I sit "ON" the seats and the side bolsters are no higher than the center section. If I were to do it over (and I might once everything else is done on the car) I'd shave at least half-inch to an inch of foam out of the center seat bun for the base of the seat.
66 Mustang GT Fastback
70 Mustang Mach 1
72 DeTomaso Pantera
69 Cutlass Convertible