>If it's hard to provide the service to them, the price should be correspondingly higher to discourage use of the bus except when it would be efficient to use it.

You've got the economics completely backwards. Running a bus route has a fixed cost; where the fares collected fall short of that cost, it may be necessary to subsidise that route. You actively want to encourage more people to use that route, because it reduces the level of subsidy necessary. People who find the route merely convenient help to fund the service for people who vitally need it.

Bus subsidies are often remarkably cheap on a per-journey basis, because the service would only be modestly unprofitable without the subsidy. Paying a few pennies per passenger to make a service profitable is vastly cheaper than paying for taxis to ferry all of those people to their destination.