At least you’re lucky to have Ross/UMich in-state. You may be able to get a scholarship for Kelley that brings it in to your price range. McGill a possibility, especially if you can knock off a year with AP credits. If you don’t get those 2-3 schools to work financially, honestly, your best bet is to go somewhere cheap/free and use the saved up money for a 1-year master’s at a target school.
BTW, an MBA wouldn’t be required if you want to stay in IB but would if you want to change careers (many bankers do; IB really is a punishing career and not worth the sacrifice, IMO).
Also, for IB in London, studying a non-quantitative subject from LSE/Oxbridge would be fine (might also be true for semi-targets like Edinburgh/St. A’s/Durham), but being an American would be a handicap and it would be tough (read: impossible) for you to participate in IB recruiting in the US.