Craig,
They range in price from $100 to $1200. A lot depends on what you want to find. A gold detector isn't as good for coin hunting as a coin detector, and a coin detector isn't as good for gold hunting as a gold detector. Then if you're a relic hunter there are detectors specifically designed for that, which also isn't as good for coins or gold.
Then you have the multi-purpose detectors that can do well at coin hunting, relic hunting and prospecting. Of course those are the more expensive ones.
I get my detectors from off either ebay or T C Detectors out of Connecticut. On ebay you can get some pretty good buys. If you want a good source to learn about the different detectors, try going to
http://go metaldetecting.com great info and the forum lets you ask all the questions you want to some very knowledgeable detector users.
I can tell you although detectors can be expensive, if you just get out and use it, it will pay for itself easily in a short time