To tell what you're getting - you need to look closely at model numbers on the bottom of the device, or package labels...
Apple TV (3rd generation) Early 2012 A1427/A1469 -- A1469 is AKA the 'Rev A.'
Apple TV (2nd generation) Early 2011 A1378
Apple TV (1st generation) Early 2007 A1218
Identifying Apple TV models
Price? your favorite search engine will get you options...