We've grown accustomed to paying only a dollar or two, if anything at all, for most iPhone apps. I don't expect that will change much going forward, but I do anticipate more successful higher-priced apps for the iPad. As Wade points out, Apple will lead the way with their $9.99 iWork apps. Although you could argue any one of the iWork apps is much more powerful than the typical iPhone app, I think the additional display surface on an iPad (vs. an iPhone) will lead to opportunities for richer applications and content content.
Publishers currently spend a lot of time trying to figure out the best user experience on the smaller screen. Reference material in particular is tricky because you want to pack as much into one screen as possible. Not only is that less of an issue with the iPad, the larger display lends itself to some clever things publishers will be able to do to enhance that smaller-screen content.
Does anyone remember VH1's "Pop-Up Video" series from several years ago? They took old videos and added value to them by popping up bubbles of behind-the-scenes info. No matter how many times you saw the original video, you learned something new when you watched the enhanced pop-up video version. Interesting factoids as well as silly trivia were added to the original videos and they were fun to watch.
Now imagine the same thing added to the small-screen version of a tutorial or reference work. (Btw, I'm pretty sure Pete Meyers described something like this in his recent TOC session.) If the original format worked well on the iPhone's screen, why not make it even more powerful by adding richer functionality on the bigger screen? This pop-up option is just one way to add value and I'm sure others will come up with even more compelling enhancements. Ultimately though, you'll be able to offer one product for the iPhone and something that builds on that same framework of content for the iPad. Done properly, and if enough value is added, it's easy to see where the latter could be higher-priced than the former.