well, apparently this is news to some of you: Asking a person how much something cost is rude. As has been said, it's like asking how much they make (which is also very rude). If you like the car, say so. If you want a supercharger yourself, say so. But don't ask about money. It's tacky, makes the other person uncomfortable, and there is absolutely no reason for it.
If you have no clue how to find out the cost of a supercharger for a Mustang, you are an idiot, and simply telling you the cost won't help. If you already know the price range and are simply looking for a place to get yours installed, say so before asking who did it, what was included, and what the rate was. This idle "so what did that run you" questioning is something only rubes with no manners engage in. Nothing says "I'm an uncouth hick with no social skills" louder. Accept the fact that not everybody gets a hard-on bragging about how much money they spend, and very few get a hard-on being asked about it.
This applies to everything, not just superchargers. Develop some manners and bite your tongue next time you are tempted to ask the cost of somebody else's toys. Ask where they got it, or how long they've had it, or where they work, but please leave money out of the discussion if you wish to be seen as an adult by the rest of the world.