There are actually two types: Shrek ears which are just the plain ears on a headband and Fiona ears which also have a little crown on them. (She is a princess after all )
If I remember correctly, they sell for $5.99 at the Shrek gift shop at the end of the attraction but when we were there in late May, I saw them at two other locations in the park selling for 2 for $10. This was at the last small open air gift shop as you're leaving USF and also at a street vendor on the path toward BTTF and MIB. If you'll be buying more than one set, you may want to search out the "twofer" price.
Do they still sell them at Walmart? I got a couple of Shrek ears this summer some time but I don't know if they are still selling them....anyone seen them lately?