I leased my bimmer from a Michigan dealership in June of this year. Listen to landshark on this one. It is imperative that you negotiate the lease based upon the sales price of the car, not the monthly payment. You negotiate the sales price, just as though you were buying the car for cash. That sales price is popped into a formula, along with residual and money factor, to produce your monthly payment. Since the residual and money factor are often non-negotiable, you basically have to negotiate the sales price to get any kind of movement.
This is what I did, and I thought it was pretty helpful:
(1) I went to edmunds.com and checked out their pricing information for the car I was looking to buy (in my case 2003 525). You can select the exact configuration (options and packages) as the car that you are looking to buy. The edmunds site provides you with the MSRP, the dealer's invoice price (dealer's cost), and will also give you an approximation of the typical sales prices in your area.
(2) shop that sales price around, then shop it around some more.
If you are in the Detroit area, then you are a bit hamstrung b/c the 4 dealerships are really owned by 2 people. Don't let that deter you. I had a salesman at Erhard Farmington bidding against the salesman at Erhard Bloomfield. Then I took their best offers and bid them against the dealerships in Grand Blanc, Lansing, Ann Arbor, and Toledo. You just have to be patient and keep chipping away. I knew I was at rock bottom, when finally no other dealership would beat or match my latest offer. When you reach that point, you just have to put your deposit down and drive away with your fantastic bimmer.
It takes time and patience. You're looking for a 2004, so I'm guessing that there isn't going to be much movement, if any, b/c it's the beginning of the model year. When I was shopping for my 2003, the salespeople were falling over themselves b/c they were trying to clear out the remaining 2003 models. Whatever you do, make sure that you take your offer to another dealership to give them a chance to beat it. When you show them that you are ready to buy (ie, "I have my checkbook, and I'm ready to fill out the financing application"), it's amazing how interested salespeople become in your business. Also, conventional wisdom is that end of the month is the best time to buy, b/c the sales staff is trying to hit their quotas.
Sorry for being so long winded. As you can tell, I get into this stuff. I guess it's just the competitiveness in me, b/c I always think that dealerships are trying to rip me off.
Good luck, and let us know how it goes. Remember, once you get your bimmer, show us some pics!