Disney always used to control sales of them very closely - pull them off the market after a few months and not rerelease them for years, to keep the price up. You could never get one for less than about �12. They seem to have changed their strategy now, though, and they're much cheaper:
http://www.play.com/DVD/DVD/3-/240841/0/-/Prom o.html?ob=price&sp=1&dpr=240841
So you're not going to make any more money out of them than for any other DVD now. Sell them now, but it's probably too late already to make a fortune out of them.