It costs a good bit, but not too much (my setup cost just a bit more than $1000). Get Koni shocks and H&R springs. Its a really good combo and the konis are adjustable, but for our cars you have to take the shock out of the car to adjust it (it is not the knob adjusted type). I'd watch for making sure your wheels arent too big (to avoid bottoming them out on your wheel wells, but with 17s and smaller you should be fine). Also, make up your mind on the spring pad size and firmness setting for the shocks that you want before you get them installed.
To really answer your question we'd need to know what you want after you have it lowered. just a low look or better handling? how low do you want it? How big are your wheels/the wheels you want to put on it?