I'm planning on paying cash for a GT, but I looked at the DE lease as a way to get an earlier delivery, get the $7.5k credit, and have the option of walking away from the car at the end of the lease (if I don't want to buy the car, for cash, at the end of the term). I've never leased, this site was very helpful:
https://www.leaseguide.com/lease08/ (along with some posts on this forum). As they explain it, there are two components to the payment: a depreciation fee, which reimburses the leasing company for the depreciation, and the finance fee, which is the interest they earn. The depreciation fee is basically the price of the car at the start of the lease, minus the price at the end, paid monthly. There's no explicit markup here, ie no "profit" built in for them. Then the finance fee is effectively charged on the average cost of the car over the term, which is dropping due to depreciation. So interestingly, if you're going to buy the car at the end, you hope they set a lower residual; the cost of the car is the same in the end, paid more in the monthly depreciation payments and less at the end when you buy the car; but you pay less in interest because it's calculated on a lower car value towards the end of the term.
The DE lease calculator doesn't provide info on residual value and money factor, but I took a guess at effectively a 6.75% interest rate (the same as their loan) and a typical depreciation of 20% the first year, 15% in following years, and tried to match the payments the calculator shows for different lease terms. I had to make big changes to the depreciation to come close to their payments (more in the first year, less in subsequent years), so I don't have high confidence in these numbers, but I found if I did the 18 month lease to get the lowest financing cost, I'd pay ~$10k in interest. So not worth it to get the $7.5k. This was with the default down payment. There is a discussion somewhere else on the forum about a higher down payment - there was a comment made that paying a higher down payment was not a good idea. I can't recall/reconstruct the reasoning, I think it had to do with the insurance payout if the car was totaled. I'd appreciate if if someone can comment.
The lease calculator will only let me go up to about $41k for the down payment.
My SA couldn't give me residual value and money factor. They have it obviously to do the calculator. I asked him to call me if he does get those numbers and the DE order book is still open. I'll redo the calculation with real residual / money factor when I can but I expect the result won't be that much different.
It got easier for me to get a handle on all this when I started to think of leasing as: the leasing company buys the car, you pay them for its use during the term, and then they sell it. Then there are details (fees) on either end of the lease term. This helps make sense of other comments people in the forum have made, such as not being allowed to get custom licence plates.