I think you are correct.
Have you read this?:
https://www.tax.ny.gov/pdf/publications/sales/pub839.pdf
"Once a lease has been entered into and the customer has paid the tax due at the inception of the lease, no refund or credit of tax paid is allowed to either the dealer or the customer (except as explained below) if the lease is terminated early, even if the lease is terminated because the vehicle is
destroyed in an accident."
This would suggest that once you do the buyout, you will pay sales tax on the entire buyout amount since you are getting no credit for the previously paid taxes.
However, I don't think the difference will be $1500. Here's why:
I am assuming for this simplified example a 7.5 state and county combined sales tax. You were correct in calculating the lease sales tax on the lease cash. So for the lease term, 7.5% * (~$49K agreed upon value - ~$29K estimated residual) = ~$1,500 as you stated above. If you did a buyout right away, the buyout amount would be around $40K, so as the buyer you would pay 7.5% * ~$40K = ~$3K. You would have paid roughly $4,500 in taxes.
If you did a purchase transaction instead you would be taxed 7.5% * ~$50K =~$3,750. So the actual difference in sales tax is more like $750 ($4,500 - $3750). I don't think it will be $1,500 difference because the buyout price will be $~10K less than original agreed upon price due to incentive. However, don't forget that lease acquistion fee is $650 and buyout fee is $300. So you are still looking at a $1,700 "inefficiency" in the lease route. Without consideration to finance costs, the $3,320 "benefit" of the lease cash, is quickly reduced to ~$1520. The difference will likely get reduced a little more because the lease money factor results in higher interest rate than a typical purchase loan for those one or two payments you will be required to make. You would also likely incur some additional fees related to becoming the owner (registration fee, title fee, tags, etc.). Another consideration is whether lease buyouts in NY can be completed directly with Kia or are required to go through the dealer. The latter would be a deal killer for the savings you are talking about.
I did a lease in 2018 because there were no cash incentives that strictly applied to purchasing at the time. So $7,900 lease incentive was a big deal. My lease was taxed on inception for the entire agreed upon value, but in MD any additional taxes are waived when the lessee purchases the vehicle. I only had to pay a couple hundred more in registration, title, tag fees so it was an easy decision for me. But in your case, I'd also be on the fence as to which way to go. I denfinitey would not do it for that modest amount if I in any way had to go through a dealer for the buyout (I'm not sure how KMF/NY handle).
One last consideration... what would the purchase finance rate be by taking Kia financing? What interest rate would you be able to get if refinancing at lease buyout?
