that is not a common occurrence, Galactica. Both ponies in that auction normally go for much more than that. It took me ages to get my Mummy pony, and when I first got one it cost me around $30, I then later bought one with the babies for $75, and sold my first one for $35. Neither worked. The other pony in that auction can go up to $20-$30 with her key.
Who knows why that one didn't go very high. It's a mystery. It wasn't listed well, but the photos were ok. Maybe people have run out of money because of Christmas. Fact is someone just got an amazing deal. You can't judge the value of a pony on one Auction.
Does this mean she shouldn't have been used in the art. No. Is that piece of art worth more than the pony. Yes, probably by a mega tonne. But it doesn't change the fact, that that particular pony can be hard to find.
Here is a link to completed surprise twins mummy pony