I'm wondering at what mileage should I replace the supercharger belt and accessory belt? Also, what is the estimated cost when the dealer replaces them?
Well, first of all you can easily do it yourself. Takes 5 minutes. The belts cost between $25. - $35. depending on where you buy them. Rockauto.com is the cheapest I believe for Gates belts, which is what you probably would want. I am assuming your car is stock? If so you can check your Owners Manual as to when Ford recommends it be replaced. IMO since they're so cheap, do it once a year. The last thing you want is a belt to break when you least want it to.
For the dealer to do it you're probably looking at $50. for the belt plus another $30.or so for the labor. Trust me, it is very easy to do. The hardest part is releasing the tensioner with a 1/2" socket wrench because you have to maintain pressure while you start removing the belt and then do the same thing to get the last of the belt over the pulley or idler.
Suggestion, since you haven't done it before. Find someone nearby who has a Cobra and would be willing to show you how to do it. If I was nearby I'd gladly do it. Once you see how it is done you'll agree that it makes no sense to have the dealer do it.
The accessory belt is not that easy to replace. Remember it drives your A/C Compressor, Power Steering, and most important Waterpump. You have to remove the auxillary crankshaft pulley, move the power steering reservoir to the side, remove A/C muffler bracket and put aside, and then finally relieve tension on the belt and remove. Do a search of lower pulley removal. You will see quite a few people asking how to loosen it up, to remove it. Maybe someone in your area will be able to give you a hand changing the belt. As Cobra Bob said the supercharger belt is a snap. Best of luck.