I did a search in the PC archives over at the Arena and apparently a loose one went on eBay for $40. Some are saying $30-$40 loose. I'm not sure how much a MIB one would go.
I, however, managed to snag a MIB one on eBay for $8 a couple years ago with a BIN. I also got the MIB Birthday Sweetberry from the same seller for the same price with a BIN. Those are some of my most prized ponies. :allecto: Love 'em.
So yeah...these are very sought after.