How much does it cost to supercharge a 2005 Mustang GT?

If you want the finest supercharger on the street that money can buy, then you need to seriously consider Whipple. Plan on spending about $5K. They're instant boost. Because they're a screw blower, they do not heat the intake charge. Since they don't heat the intake charge, your boost to octane threshold goes up tremendously. Step down on your spark plug heat range at least one step. Try the Denso iridium power and don't change the gap on them out of the box.
If you need help in selecting your spark plug heat range, I'll do that for you.

Going to cost at least 3 grand just for the kit, then labour could be another grand. Engine work would depend on how much boost/power you want. Most descend running engines can handle 6 or 7 psi without strengthening the bottom end.

well say the s/c is roughly 3500 and you have someone intall it for you maybe around 5000 all together. BUTTTT save some money and intall it your self