I decided to put a bit more money down on top of my trade-in (and because of the California rebate).
If you have a trade in with equity in it, and are putting money down, its not actually "403 a month" . That may be your payment but thats not what you are paying. If you had 2k in equity and put down another 1.6k (making up numbers) that would be 3.6k in cap cost reduction. Divide that by lease term of 36 months and thats 100 a month, so it would be 403+ your 100 for 503.
I suspect that if your payment is 403 with 15k lease term for 3 years, you had substantial worth in your trade in, and / or added additional money.
I am a pretty big fan of leasing when it makes sense... leased BMWs for my wife and I (2 separate ones) for 12 years. So, I am not anti leasing, but its generally not a good idea to put a trade in on a lease unless you are trying to get out of a car with negative equity.. which you likely are not doing with a 403 a month payment.
Anyway, its a bit misleading to put a lease payment out there that has a huge cap cost reduction in it, because one can make their lease payment whatever they want with a large enough cap cost reduction, and trading in a car can provide that.