Are souls really in that short a supply? Dragon hearts too for that matter?
If you're willing to accept infinite primes on infinite worlds, the extended mutliverse and hordes of adventurer's running back and forth across the world with magic blazing from their fingertips, I figure the prices are exactly right.
I don't see the dragon's heart as being the entire heart for starters, more a small fragment of a preserves heart. While killing a great wyrm may not happen every day, there's probably enough adventurers on enough worlds to keep the supply up.
And any demon, devil or deamon that looks at the constant influx of dead souls roaming his plane and doesn't think of something interesting to do with it is really just letting the side down.