I have no problem with Monte charging what the market will pay for this book. In fact, I like that there are high-end d20 products out there. I don't buy many of them, mostly because I'll never get the chance to use them, but I think it looks good for our hobby to have high-end products. It reinforces the fact that the stereotype of the teenager in his parent's basement is wrong. There are plenty of gamers working good paying jobs who can afford this.
With WLD cover priced at $100, this makes sense. WLD is a great book, and it sounds like Ptolus will include even more bells and whistles. If Amazon discounts it, I might go for it.
Seems to me Amazon is a big factor in book prices these days anyway. I recall from my economics classes that coupons and sales are a way to for a company to play both ends of the supply and demand spectrum. If a company can sell a product at full price to the people willing to pay full price, and at a discounted price to bring in those consumers who wanted the product but were not willing to pay quite as much, that is just smart business.
It worked on me with WLD. I waited for the big discount at Amazon, and after seeing the book I think I would have been happy paying cover price. As Buttercup mentioned, many DVD collections go for this price range. I would rather pay $120 for a book, some maps, and a CD, all with content new to me, that for a few seasons of a television show I have already seen.