It really depends on what kind of drop you want, and how much quality you want in the hardware.
You can pick up some good struts and springs for about $300, and it will cost somewhere between $150 and $300 for labor from what most people have said here when it's been asked in the past.
If you start talking about big drops, you're getting into the need for camber kits and stuff, and you're talking another $100 onto that.
If you want to replace your strut boots and the various urethane and rubber pieces that are down there you're talking maybe another $150-$200 for all of them.
