The keychain is authentic.
No one can tell you what price an item is worth. It depends on the rarity of the item, how badly you want it, how many others there are to choose from, etc.
Often, Coach items will sell for more than their original retail prices because the items are no longer available and in high demand.
Only you know how much you want this item and what you can afford to pay for it.
IIRC, that style keychain (#1733) is about 4 years old and a search of ebay for that style didn't pull up any listings. So my guess is that you'll probably have trouble finding it, PERIOD, let alone at a lower price.