Is that the yellow gem symbol SLH? I don't actually see her very much. I don't think I've ever seen one here second hand...
Yeah, google says I'm thinking of the right pony. One loose one sold on Ebay UK recently for just around £10 so I would suggest maybe roughly $12-15 wouldn't be out of the ballpark.
But she doesn't show up a lot. I have her in my boxed ponies I think but I don't remember ever seeing another since.