One of the things that fascinated me about reading Monte Cooks campaign at his site was that idea of making the fantasy elements of the world out of the rules, instead of making the rules fit some idea of fantasy. Not necessarily my thing (although I did try it), but it's something I keep in mind when I read through most things these days.
I'll admit my first reaction upon reading those prices was pretty negative. Then I went away and thought about it for a bit. Souls have been a trade commodity in DnD games for a while - I still remember references to larvae/worm things that were the embodyment of evil souls being sold to liches and demons throughout the multiverse in earlier editions. Hundreds and hundreds of the things were scooped up and sent wherever they were needed.
I didn't have a problem when the souls were being traded in larval form as part of some nameless, shapeless web of evil, but suddenly someone puts a market price of a few hundred gold and I'm shocked? Low-powered souls probably are a pretty common commodity in a world where demons and night hags can just go scoop a few larvae out of the pool. Whole dragon hearts should be a valuable thing, but as I mentioned before I have no problem with tiny scraps being worth a few gp.
Where I really made my peace with this was when I started thinking about access to the compononts. Just because someone's willing to sell you a soul for a few hundred gold, it doesn't mean you can suddenly travel down to the local market and pick one up. IMC, the people who deal in such things are hard to find, often hidden in strange extradimensional pockets where they can deal with beings from a hundred worlds. They have easy access to their wares - no-one asks where and the chances of finding out are slim. The big-bads of magic know where to go to find stuff, but the lessor casters dont.
In many cases the cost required to get to the trading post where a souls available is going to eclipse the cost of the soul itself.
