I have bought watches to part out in the past when I needed a couple of specific items for a franken. You just need to be careful what you pay. IMHO, the prices you get for standard parts have fallen at the same time that the second-hand gens seem to have risen in price. It is much more difficult to do it. Now the only way I do it is to part it out in advance of the purchase. My conservative rule of thumb is that you need to buy a gen hand wind for under $3,000 for it to work (ignoring what you were willing to pay for your parts). I would guess the following prices on parts (but I am conservative):
E movement - $1,000
Tobacco Dial and Hands - $800 (on a good day)
Caseback - $300 - 400
So what would you pay for a full case set with crystal and no back? $1,000 - $1,200? The total is then approximately $3200. And of course you should never take apart a gen.