How much to lower a 2000 toyota celica gts?

Shocks are for rebound, springs are for lowering. For example, you can retain your stock shocks, and fit a set of lowering springs on top to lower your ride height. The bad side to having a stock rebound and lower car is that dips/speedbumps can lead to scraping, bottoming out, "omgwtfmyoilpan" moments, etc. Some aftermarket manufacturers have shocks that allow you to adjust rebound 3 ways, 5 ways, etc. that will allow you to "stiffen" your rebound, at the cost of ride comfort. Changing over to those could change the height of the ride without springs. For springs, not everyone is the same... so you'll want to look at what the manufacturer says it drops, and if the springs are "stiff" themselves. There's a LOT of factors that also add into just lowering the height of the car. If you're aiming for flush, hellaflush, tucked, low but still streetable, etc etc etc... DON'T cut springs. EVER.