Well if you're buying one direct from a place like Wacom.com or a large computer store they can easily be up to $200 and more if it's a Wacom Intuos tablet. (On wacom.com they're $329.99 for A5 size D: )
On eBay you can get them MUCH cheaper but you do have to be careful waht you buy and there's really no point getting a cheap alternative. Wacom is the best by FAR and the only one worth your money. I bought a cheap alternative for £80 and it broke it 2 weeks...
I currently use a Wacom BAMBOO One (bought on eBay) which only cost £70 but on wacom.com they're $79 which really isn't bad considering how expensive they can be.
You'll probably find people saying a Wacom Intuos 3 is the best to get but unless you're a graphic designer by trade the BAMBOO works perfectly and is more than enough.
The Wacom BAMBOO page on wacom.com -
http://www.wacom.com/BambooTablet/bamboo.cfmI really, really recommend getting one for anyone who does artworks digitally as it makes everything so much easier and you'll very quickly find that you have better control over lineart, detail and shading. You just have to remember a tablet pad doesn't work in the same way as paper. XD
Also: As far as I know, the Wacom tablets come with extra art software. Don't hold me to this though as I've never bought one from Wacom directly, only off eBay, but mine came with a full version of ArtRage 2.