Lease Buyout Scenario

The lease on my X3 M40i is up in a couple of months, and I’m looking at moving into an X5. Residual on the X3 is $40K, and I’m seeing trade in and private party valuations ranging from about $45K to $52K, so it seems I’d have some equity if I buy it off the lease.

Thus, I’m thinking it might make sense to buy the X3 off the lease first and trade it in towards the X5. I’d get hit with 6% sales tax on the residual ($2400), but if I turn in the X3 I’ll have a few thousand miles of overage plus the disposition fee, which offsets the tax by $1200 or so, still leaving, let’s say, $3500 to $10K in equity.

Am I missing anything obvious where I’m better off just turning in the lease?