For me the issue isn't the cheapness of public transport, but the availability. Where I live (outer eastern suburbs of Melbourne) the public transport system is a hopeless mess. Buses are the only way to travel unless you're going radially to or from the city centre (I do use trains to go to uni when I have to travel during rush hour times), and they are unreliable, infrequent, and take long winding routes through backstreets that add huge amounts to travel time.

When I was living in London I was amazed at what a joy it was to live in a place with such a well-run, useful public transport network. It made me sad when I came back home.