Well, yes and no. While you can end up paying nearly the full figure price to get parts, that's not because they're worth that much, people that sell parted out figures inflate individual items because they're hoping to get the most money and if some things don't sell, charging more for others and getting it can cut your losses.
I saw an eBay auction for $15 plus shipping to get the T-1000 stand, my ass that stand is worth that much, it's just a little plastic thing, there's nothing to it, and it's a standard design used over and over.
Buying figure parts is a lot like fixing your car, it costs more to put it all together after the fact than it does just to make in it's original packaged form, it's insane.
That's why I'm buying 3 full T-1000 figures to make mine, I'll have parts left I can either bash into something, or sell off, but to get just 1 or 2 in order to make 4 unique figures just isn't cost effective with the needed parts.
Unless you're looking for one or mabye 2 pieces of a figure set and they're something besides the head or body, it's not very cost effective to buy parts.