I know a guy that went to Chicane (in Torrance) to get springs installed on his 05 or 06 GT and I think they charged him $250. A day or two after his install, he had to take it back because of some popping noise coming from the front suspension. Not sure what became of it.
Don't forget, you'll have to get the fronts aligned after your drop.
I still have to take mine in, haven't got the time. Not really in a rush because I've only driven around 10 to 15 miles since I lowered it.