Not what you've spent on it!! We all know the headache involved with selling cars especially in theses times, save the stress keep you z and save up for a month or two for a down payment on your new car. I'm always surprised how everything will just fall into place if I let it. I have a motto, Once I pay a car off I keep it, no matter what, no regrets, but everybody does it differently.
This way you don't have to buy another Z in the future just a TT front clip and go to town.