You can buy one for $50 or so as pulled from service off EBAY. These usually require a little work but nothing hard - just replace batteries and clean it up. For $50 you get a 1GB flash drive and XP Embedded.
$200 is what I sell them for completely refurished with new batteries, stylus, rubber guards, docking station, 30gb internal hard drive and XP Tablet Edition.
So that's your price range $50 to $200 based on condition and accessories.