If I may, I would personally really like the app to look like the default Calendar App. Why? Because it’s clean, easy to use, and intuitive. Keeping the same type of interface would let people concentrate on the budget and not the interface.
By the way, my strong personal suggestion: keep this free. We already have to pay for the service and, even though I realize that there are costs involved in developing and maintaining the app in the app store, I wouldn’t like to pay a lot for a new interface to a service I’m paying. Then again, paying $0.99 wouldn’t be a big deal, so if you feel you have to, keep it at that :)
Also, when you thought of this originally the iPad was just a dream – now it’s a reality that you may want to deal with. The iPad offers much more screen real estate, so it’s a definite plus. If you charge for the app, make it “universal” (for both) and you’ll make everyone happy!