I wanna say, like $10 - $25?
They are sought after by both pony collectors and McDonalds toy collectors, so it can also depend on where they are listed on eBay (if on eBay).
Edit: Just checked eBay's "Sold" section and found pictures of on MIB! Neat, I've never seen that before!
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loginInteresting--the bit of text I can see suggests they are actually charms to hang from your pocket, not bookmarks.