18k per year is a lot in terms of mileage, I'm not sure what residual they'll use to calculate that, honestly with that much mileage leasing probably doesn't make as much sense as buying it out right would and selling it when you're ready to move on. You aren't getting the benefit of a high residual and even though your payments would probably be close to $1k/month, at least when you're ready to sell the car you'll get some of that back.
It sounds like you could probably still get a bit out of it, without doing all the #'s ~$750 sounds like it'd be close. Ask to see what money factor they are giving you, along with the cap cost of the car (IE what they're actually selling you the car for). Just keep in mind at that point, you're putting almost $33k into it after 36 mos, which means purchasing it outright and selling the car afterwards would probably be a better bet.
Also keep in mind that if you put 54k on the car, you'll probably need to purchase (or want to purchase) a pro-rated extension on the warranty to cover your ass in case anything goes wrong at 52k.
2013 335i M Sport /// Alpine White/Coral Red