Formulas or rules regarding End of Life for vehicle Are there formulas or rules when keeping a vehicle is no longer financially beneficial?
I understand the below needs to be considered:


Every time a purchase is made, sales tax will need to be paid.
Keeping a vehicle longer reduces the number of sales tax
transactions.
A vehicle, especially new vehicles of certain models,
depreciate quickly - exponential depreciation.
The retail book value comes into play as well as the age of vehicle on whether a repair should be made; 10%, 20%, not certain.
There is no one general standardized formula for the value of a car. That is not how prices work. The price of a used car is going to depend on what other people are willing to pay for it, not some depreciation formula that you can just plug numbers into. This inherently means there is some uncertainty in trying to determine the value of a used car.

With that said there are resources out there to approximate what people generally are able to buy/sell a used car for such as the Kelley Blue Book which records the average selling price for models of used cars after a certain number of years. These resources should give you an approximation for what a used car could go for but it will never be exact and is location and time dependent.