On eBay, older used iPads are priced anywhere from $120 to $600, as of April 2015. Original iPad models are the least expensive, at an average price of around $120.
Used iPad 2, 3 and 4 models are generally between $200 and $250, but can be priced higher depending on the storage space and whether they have cellular capability in addition to Wi-Fi. Used iPad Minis tend to average around $185. IPad Air models are the most expensive, and even used ones can be priced as high as $600.
Apple also directly sells refurbished iPads for anywhere from $209 to $629. Used items can be cheaper than refurbished items, but refurbished items are more reliable, and they are typically in better cosmetic condition. Additionally, refurbished items come with a warranty, whereas used items most often do not.