Roads are paid for via taxes, mostly on gas. This is widely accepted, to the point where nobody even thinks about it. If I drive to work, I only indirectly pay for the roads via the gas tax. That's $0.45 per gallon in Minnesota, including federal and local taxes.
So why am I expected to pay $2.25 every time I take a bus during rush hour? Metro Transit, like most mass transit systems and unlike most roads, is expected to provide a large part (currently about one third) of its own funding through fees and fares. In order to pay the same amount commuting to work via car as I do commuting by bus, I would have to use up 10 gallons of gas every day. Needless to say, I don't. I'm actually punished financially for riding the bus, which has all kinds of positive externalities.
Part of the answer, of course, is that the money from fuel taxes goes overwhelmingly to highways, rather than transit. Why are we funding the less efficient method of transportation with more negative externalities (see warming, global) at a far higher rate than mass transit?
These prices are way out of whack with the actual costs involved. A proper price on carbon would help.