As someone who does not own a car, I doubt cost of public transportation is not a factor in its adoption. People don't ride public transportation for fairly specific reasons: wanting personal/dedicated space, wanting control of schedule, wanting to get from point A to point B faster, etc. The reasons why people *do* choose to ride public transportation are generally more dependent on financial limitations: parking is too expensive, don't own a car, tolls, etc.
To increase adoption of public transportation you need to make it faster and more convenient: no transfers, high frequency, easily understood route system, stops close to origins and destinations, etc. Where I live there is one commuter bus line and one "neighborhood" bus line that I use periodically. The limitations on increased use really come down to increasing frequency and making sure there are viable alternatives in off-peak times.