Most of the other graphics should be able to be reused. I'm not quite sure how larger screen = different graphics. A log of games on PC use the same set of graphics with screen sizes ranging from 800x600 to 1920x1080 or possibly even higher or lower.
How is the change from iPhone to iPad much different than that? Make the UI scalable and then introduce a zoom feature as needed. That's how it comes across in my mind.
This is the first deathtrap people fall into when making mobile stuff. Size is everything.
With either computers or laptop screens, you have too much space. Like when I try to reply, there's a whole bunch of information I don't need, like two rows of emoticons, formatting, "796508 Posts in 28674 Topics", all this other useless crap I'm never going to look at. This is because there's so much space that the page looks empty without it.
Use it on a standard monitor or a laptop or a TV and it's all the same thing. Nobody notices.
Now open this same page on a mobile phone. All that crap hits you on the face. A proper mobile forum page would probably have just a reply box, buttons, and one row of formatting stuff. The buttons will have to be resized big enough for a thumb, which will take up a significant portion of the screen.
Let's take that mobilized page and put it into an iPad. iPads have just the right amount of size, but all of a sudden you have a lot of ugly whitespace because your reply box is filling the whole screen. A good UI developer will spend hours/days customizing that and all other pages to be suitable for tablets.
Now that's just the first part. I'm not an iPhone developer, but I'll just say what I heard from colleagues. It's not so bad on Androids because Android is designed to work on every kind of size.
iPhones are not. They're designed with a very specific size. A lot of people complained that the iPhone 5 broke a lot of apps because the longer screen messed up some hardcoded calculations. But the software is just not designed for 'soft' screen calculations unlike an Android. You have an exact screen size, no reason to soft-code things like positions.
This just doesn't translate well at all to the iPad. Maybe things like static text pages are ok, but games will have to be massively overhauled. Porting is not easy. You'll get a whole string of problems you don't expect, and I guess some developers will vent their frustrations out by raising prices.
If people do just directly scale it up, I don't think you'll see a difference in prices. Also note that an iPad is 6 times the area of an old iPhone. Try and imagine the kind of work it would take to design for a monitor just 4 times your current one.