Welcome to MINI2.
Installation is a bit dependant on dealer labor rates, and SF/The bay area has some of the highest - in any case, there isn't much info on using the '05 pre-wiring, because there's no official kit for the 2005s from MINI even (which makes it odd that your dealer is even quoting an install... perhaps they're going to use the '03-'04 kit...)
It should be possible, but could involve some adapaters and such. Personally, I'd worry more about the looks of the extra unit in your car, then the antenna, as the antennas are really small and blend it pretty well when placed near (usually in front) of the normal antenna.
We should have info on the official '05 kit shortly (but that could be within weeks, or it could be longer, MINIs not consistant on delivery of these things) and it is expected to be cheaper. If you're not in a hurry, I'd wait a bit until that comes out, they maybe check at one of the cheaper dealers (I hear East Bay isn't too bad) for how much they'd charge to install it.
Otherwise - The Aux jack doesn't need to cost $140 to install, its like $50, plus however much they charge, which if you don't have them drilling and doing it in the glovebox, can be very reasonable. Or you can install it yourself anywhere you like. That opens you up to the aftermarket options, but sorry no help on the antenna thing, I'm waiting for the official kit myself.
Finally, you could look at XM. There's a kit fcalled XM direct that a few of the MINI2 sponsors sell, which uses the headunit capabilities with a tuner, so all you have is the extra little antenna - that's been a popular route, as its several hundred cheaper then the official MINI one, and still is a clean install. Of course, it is XM, and hence I'm personally waiting for a good Sirius solution myself.
