I don't think there is a magic formula. You can look at average monthly repair costs vs. new car payment... plus this and minus that...
But it's really up to you. You can probably find a new car that would be less worry and pain, and you can plan ahead and know how much you will need to spend each month (especially with the 10 years/100,000 mile warranties nowdays). If you are tired enough of the car and working on the car, and paying for repairs, etc. that you are worried about it all the time, etc. then that probably will outweigh that fact that the SVT is paid for.
Some people say that you should get rid of a car when a repair will cost more than the car is "worth", but I'm not sure I believe that.
Who knows. Good luck!
