The Skywarp MP is an oddity since it hasn't kept its value like the other MPs. I've been sitting on my extra one, but they still are going for less on the secondary market than their retail price of $60.
Walmart overestimated the demand for it and over-ordered them. They ended up being shelfwarmers and got heavily discounted. This may be partially the problem with its current value. Since TRU took over, those do not seem to be abundant and most people complain they never end up even seeing them show up in the stores.