how much is this worth?

Maybe a $1000 for the whole supercharger kit.


Matt

I wouldn't consider this a "kit". This is just another blower pulled off a Supercoupe. You still need to do a lot of fabrication to install this on a Mustang, and there's no instructions included, so you'd better know what you're doing.
You should be able to buy a SC engine from a junkyard for $500 and get all the stuff you would need (or just do a transplant, but that's even harder).

The option with a much greater chance of success is to buy one of the commerically available blower kits designed for the Mustang 3.8.