Ivys prices are spot on...personally I think with super commons sometimes you are better trading for an equally common pony you maybe dont have in your collection - simply because by the time you have bought packaging to post her and paid paypal fees...you dont really end up with much to spend on another pony!
Or you do what I do which is keep them "just because" and end up with a drawer full of doubles you cant bear to part with for some kind of compulsive reason lol.