No, SAT II tests are no where near the euqivilent of an A level, a closer comparison would be AP tests. Some of the previous posts talk about the differences. Basically, as a UK student, you'll have much more advanced knowledge of your a level subjects than a high school graduate, but they'll have studied more subjects. Most UK students therefore enter US universities as second years (sophmores), apparently.
I know that MIT charge around $32000 for tuition fees, and estimate that you'll spend an additional $8000 on living expenses. That's a fairly typical figure for the top private universities. Cos of the huge expense, they have financial aid schemes, which basically assess how much your parents can afford to contribute, how much you could earn over the summer and working part time during term time, how much you can realistically borrow and then they'll top up the rest. This sounds fairly good, but when i was looking at MIT, it turned out their idea of what your parents could contribute involved remorgaging your house, etc. You can get scholarships and stuff like that as well, but as an international student you might struggle.
On the whole, from a financial perspective, as a UK student, you're much better off going to oxbridge, unless you're dirt poor, in which case you might be able to get a huge grant from the university. A UK degree will be cheaper, even with full £3000 a year fees than a single year at an ivy league school, not forgetting that you'd have to do four years in the US rather than 3 over here.