How do you determine the insurance companies determine the pay out in the event of a total loss?

The "total loss" threshold is determined by multiplying the credible retail value of the vehicle (usu. the "Blue Book" value) by some percentage, usually 80%. If the actual dollar cost to repair the vehicle would exceed this threshold figure, the vehicle is then declared a total loss.

I don't believe that sales tax or other "fees" are included in the initial determination. So, for example, if your vehicle has a "Blue Book" value of $10,000 and the cost to repair it is greater than $8,000, it will be "replaced" rather than repaired. You should receive a check for $10,000 minus any deductibles plus the sales tax in your state of residence .