[strat61caster]

I'd bet $5k (as estimated above on the 'old' car being worth somewhere around $23k at best) plus $2k-$3k in fees to the dealership and government, could be higher if they con you into paying more than ~$28k OTD on the new car, entirely possible depending on the area.

So assuming you're still paying off the '17, take your monthly payment and add about $150 to it for the next 60 months. It might be a little cheaper than that but I doubt it since you can't work this out on your own they're going to take advantage of you. ($7k/60 = $117/month)

Edit: just double checked as a note to out of staters, California sales tax on cars does not get to deduct price of trade in before that gets brought up.

Edit2: If you're not upside down on the loan going to an older car could put cash in your pocket and a MT in the driveway, a '14 BRZ should be a safe bet and not too hard to find for $20k or less.