Well, based on the fact that Sandisk has an entire line of MS Pro Duos slated to hit EB along with the PSP, I'd say that eventually the price will come down. EB has the entire size line up from 1GB ($150) down (in fruity new colors!) listed in their PSP section. It's obviously going to be a big part of the PSPs appeal and I'd imagine that as other 3rd parties get licensed and in on the MS Pro Duo act, we'll see prices drop.
That being said, scarcity drives up prices, and since most of the people buying PSPs this year are hard-core gamers, and most will be buying 512 or 1GB cards, I can see these things getting both hard to find and possibly MORE expensive. At least for the first year.
Prices will probably really start coming down next year, though.