I dont have time to do the lease calculation but I would say that this car should be somewhere in the $850/month range with current incentives and programs and $1000 over invoice.
Even with 3K negative equity that should only raise the lease monthly payment to around $950. I'd take your car to a local carmax dealer and get a quote from them to see if they will give you more because it sounds like this bimmer dealership is trying to take advantage of you on the trade.
Oh I just realized you are in TX...leasing there blows because you have to pay sales tax on the total value of the car instead of just the lease payments. So...I'd say its about an extra $3000 of taxes over a normal lease which means the payment should go up another 100 or so per month...$1100 should be about right if you are willing to let them walk on you for the trade...
I'd tell them to call it an even $1000/month including tax and see what they say...that deal would essentially be $1000 over invoice and even money on what you owe on the trade-in. minus all incentives (assuming you qualify for loyalty if your current loan is with BMW) also, are you a USAA member? that would be another $500 off.
Current Car: '16 SUNBURST GOLD F80 ///M3
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Former Cars:'13 SANTORINI BLUE E92///M3
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