I would not recommend getting used twins because after jibbing and doing air for one season the camber can really get messed up.
Man, any ski(old school, new school) with it's camber gone is like a dead ski. U wouldn't even be able to pop as high as you want.
Pay the extra, even if you have to force your dad to use his money underneath the bed, and go get a new twin.
U get what you pay for. Rules of thumb!