I would expect the Marketplace to cause Apps to cost more money. The Marketplace is designed primarily for people who aren't members of PPCGeeks and XDA, and who don't want to or know how to find and install apps. I think most of us here are happier with free betas and alphas versus a $30 Marketplace app. --- imho
However, it could turn out to be very good for developers and in turn cause a lot more cool apps to be produced.