There are three categories of applications (generally speaking) when dealing with the iPhone and iPad:
- Apps that are written specifically for the iPhone,
- Apps that are written specifically for the iPad, and
- Apps that are written for both the iPhone and the iPad.
Not all developers are offering universal binary applications (those written for both the iPhone and the iPad). I have not run across any universal binary applications that I had to pay extra for when I had already purchased the iPhone app. However, I have run across developers that have released apps specific to the iPhone and the iPad, and in this case, I have had to pay for the iPad app when I already purchased the iPhone app.
You can, as llofte pointed out, simply install the iPhone version of an app onto the iPad. But be forewarned that the results are likely to be less than stellar. If the app is not universal binary, it will have to be scaled up to fit the iPad screen resulting in poor graphics.