If you want just one app at one price, you can submit the app again (in a subsequent version) as a Universal app that runs natively both on iPhone and iPad. This kind of thing happens quite often, as developers expand their horizons...
If you submit an iPhone only app and an iPad only app they will be two separate apps with two different names and two separate prices. People do that - the New York Times app works that way, I believe - but it isn't as common. For one thing, it's confusing, because the iPhone version will run on the iPad, but as an iPhone app, so the user may not realize this is the wrong app (and it will probably look kind of crappy).
GoodReader is another well-known example where the iPhone version and the iPad version are two separate apps: see http://www.goodiware.com/goodreader.html