A pulley swap is likely the best, bang-for-the-buck mod that you can do on the Ford GT, IMO.
I agree with Shadowman that it is much more important to be confident with the competency of the shop than with the rate quote. If the shop is inexperienced (and it is likely that MOST Ford dealerships are inexperienced with GTs), then they might give you an inaccurate quotation. Too high, and it may tick you off, too low and they may be rushing to do the job in the time allotted.
I could easily imagine that Shadowman, the GTGUYS, and I all have different approaches. No right or wrong here, but each of us likely have our preferences.
For us to do a pulley swap, we do the following:
1. Remove MAC subwoofer from interior and remove access panel to gain access to pulleys, tensioner and general improved (belt tracking) visibility.
2. Relieve tensioner, and remove belt from SC pulley.
3. Remove airbox and airbox boot.
4. Remove all SC plumbing (the EGR and one of the hose clamps can be a pain)
5. Remove SC bolts and then carefully remove SC from engine.
6. On clean workbench, drain the SC fluid.
7. Using a relatively unique pulley puller, remove pulley from SC.
8. Remove snout from SC.
9. Using a press, install new pulley.
10. Re-install snout using an appropriate sealant. Torque to spec.
11. Add new SC Oil to specs
12. Install SC on engine
13. Torque SC to factory specs - and factory torque pattern.
14. Re-install hoses and EGR components
15. Re-install belt
16. Check belt tracking
17. Re-install interior components.
18. Re-install airbox boot and airbox.
19. Change sparkplugs (I too prefer the TR7IX's)
20. Start engine - confirm "normal" idle characteristics & belt tracking
21. Install MAFia (if specificed by tuner)
22. Install Tune to PCM.
23. Re-start engine to confirm proper operation.
Hopefully, I didn't miss a step in my description. My point is that there are a lot of steps - and a lot of areas where it helps to have experience and/or patience.