Texas State has a great bus system. You pay for it in your tuition, so there is no upfront fee to ride them. San Marcos also has a tram service that runs to campus as well as areas the Texas State bus routes don't include. Certain times during the day, the buses are packed full of students, and other times, there are hardly any students on the buses. Buses run from as early as 7 a.m. to as late as 11 p.m. They are pretty punctual as far as getting to the stops on time, which are spaced at about 10 to 45 minutes depending on the day (after 7 p.m., the buses don't run as often).
There are also buses that take you to Austin and San Antonio. These buses do cost money: $40 for a bus ticket worth five round trips or $6 per ride. A little bit pricey, but, so is gas. The San Antonio bus will even drop you off and pick you up at the Tanger Outlet Mall.