There are some fees, $35 I think plus registration which is $50-$70, but then there is the 6.25% sales tax based on 80% of book value or actual price, which is the biggest part of it.
Here is their value calculator:
https://www.txdmv.gov/standard-presumptive-calculator
If it were me, I would register it instead of using temp tags and not having title in your name, trying to leave the country with it. Might get hassled a bit more, border patrol might suggest its stolen, but I haven’t really ever driven over the border since I was a kid.