Depending on how much it costs you to get it installed, I'd say anywhere from $4000-$6000. Shipping was the only CHEAP part of the whole thing @ $56. haha
You could PROBABLY install it yourself if you're comfortable working with engines. The kit is set up very well. It comes with everything you need to install and a book with detailed pictures and instructions. We had a question today when it was getting installed, I came home, called Dave at EE and he called my mechanic back within 10 minutes.
You're going to spend a LOT of cash, but the way it feels when you drive it down the road is worth every penny (to me anyway). I haven't had a car under 300 horsepower since I turned 16 though, so I was very accustomed to having a lot of power. Needless to say, the sport trac left me wanting. Now that I've got the supercharger on there, it's PERFECT.
If you're not ready to shell out that kind of cash, you might try one of the SCTx-Calibrators. It will firm up your shift points and give you a few extra horses. If that still isn't enough, then go for the supercharger.