Put the VIN number and mileage in the the State of Texas' own value estimator:
https://www.txdmv.gov/standard-presumptive-calculator
If I recall correctly, no matter what you pay for the truck, you'll have to pay sales tax on a minimum of 80% of the value they arbitrarily calculate. If the sales price is higher, you'll pay the sales tax on the actual sales price.
So, if the state says it's worth $ 100, and you pay $ 120, you'll pay the tax on the $ 120 sales price, but let's say you pay $ 50. You'll have to pay sales tax as if you had paid $ 80 for it.
You can use this number as some kind of rough guide. I'd want to pay less than the state's number, of course.