Depends on a lot of things. Do you want it for home demo's or pro use?
Do you want software based for your computer or a self contained unit?
Do you want a decent microphone or a really good one?

You can do anything from a cheap Radio Shack mic into ProTools free with your present setup and it probably won't cost you anything.
If you want better software will the ability to record more tracks, a decent mic, a sound booth and some decent outboard effects you can spend thousands.