Actually, a subway in San Antonio is probably cheaper to build than most other larger metros. Mostly because of the water. (Tokyo, Shanghai, London, New York city). In some cases, hard rock is better than soft rock because of its self sustainability.
I think maintenance cost would be much lower in a hard rock tunnel vs. a soft rock tunnel.
Think about NYC and their massive subway renovations that are going on right now. One of the main causes of the subway decay is the water from the Hudson river. Every day, the maintenance workers are battling small leaks and floods. Real pain in the ass, although, the nyc subway system is a must, if the city wants to continue to thrive.
The only part in s.a. that I think it might be impossible to build, is on the aquifer recharge zone. I don't know ... I'm no expert. just making assumptions
San Antonio hasn't built a subway system yet because it's just not necessary right now. (or at least not yet)