I really think you can make them for less, but you have to know what you want before you start and limit yourself to what you will actually use. Buying fabric is addictive! There are some wonderful coops that can set you up with everything you need for a good price, and if you have it all shipped at once, the shipping will be reasonable. They ship internationally all the time, and will charge you actual shipping. If you're outside the US, you're going to pay mega shipping on dipes.
You could, for example, buy pul for outers for <$5/yd, plush microfiber for FABULOUS stuffers for $10 / yd, and some lovely raw silk or microfleece for inners for <$4 per yd. I suggest doing testers with cheapo fabric, though, until you get the fit you want. It's a shame to use your good stuff on unusable dipes.
By the way, it's gotten more difficult to sell used dipes. Ebay won't allow it any more.