The Texas government is heavily Republican, and Austin is heavily Democratic, so the state has a grudge against the city. The cost for this project is to literally destroy large areas of the central city to make room. It's worth the cost in their minds to "pwm the libs."

It's hard to convey to people outside the U.S. just how psychopathically evil Republicans are.

Most of that cost is not from construction itself, not even from buying the land, it is legal (a lot of openly anti-rail legislation intended to inflate costs and risks), red tape, judicial stuff, frequent replanning due to insecure funding, political infighting ...