I believe the Texas state budget is up to about 112 billion per year.
Relative to that, I don't think that the cost to complete the wall in Texas would be a lot.
1,250 miles times $20 million per mile = ~$25 billion for the whole Texas border. But 200 miles is critical, and 200 more would do a good job.
The ~200 miles in the South (Rio Grande Valley and Laredo) which are critical (and were funded by Congress), and another 200 or so miles (Eagle Pass, Del Rio - from Laredo to Lake Amistad) would do a credible job for Texas.
Outside the miles already appropriated for by Congress, the other areas , would require less expensive barrier (~$10-20 million/mile, for super barrier), due to the terrain (lower flood control requirement).