In the app-store economy, things will start off expensive and then get dirt cheap. I expect iPad-specific apps (not just scaled up) to cost $10-15 easily at launch. I see some developers simply scaling up their apps and charging a bunch to start, but competition will bring those prices down within a few months. Eventually many developers will utilize universal apps, which allow the sharing of a codebase between iPhone and iPad optimized apps. The downside I see to this is universal apps increase the size of the app no matter what. So iPhone owner's apps would increase in size and take up more space. Same for iPad owners who don't need the (probably only marginally bigger) iPhone code in their app that they will never use (well unless they have an iPhone). In app purchases could probably be used to unlock universal app content on iPad for existing iPhone apps loaded onto the device. If developers wanted to keep the download size small for iPhone owners (since there is a 20mb download limit over 3g), they would have to release seperate apps. Since they would not want to give away an iPad app for free (as they can't tell if you've purchased the iPhone one or not already) or sell it for less than their iPhone version, it would probably cost as much (and probably more) than the iPhone app.
As for buying a second time, there are several apps I would gladly pay for again: Sketchbook Mobile, Labyrinth 2, The Weather Channel Max, 2Do, Tweetie, Distant Suns, VNC, NOVA, Band, and Plants vs Zombies come to mind to start with.