I’ll pass (for the moment) on housing and pizzas. The costs of transportation and education, however, strike me as offering a particularly easy argument in favour of a just price. The argument is related to this (as yet) neglected thread: [url=Political consequences of economic inequality consequences of economic inequality[/url].
If there is a strong social commitment to equality of opportunity and meritocratic promotion, leaving the delivery of social goods that promote those ends to the market alone would seem mistaken: the sons and daughters of the most successful will enjoy greater access and become a self-perpetuating class. A free-market aristocracy of excellence will, perhaps, serve the end of promoting excellence, but it makes a mockery of a commitment to equal opportunity, and therefore social mobility… which raises some serious questions about the character of that society.
As transportation and education costs are obvious barriers to access to the means of self-promotion, a price sufficient provide incentives to attract providers, but not so outrageously high as to exclude those of ability but not means, would seem just. (I must admit to a personal bias here, as my undergrad university was established as part of a program specifically designed to make university educations available to students in their home towns, rather than urban centers alone, and tuition was set appropriately… thereby defeating transportation and education costs.)
A just price need not mean that there is a single price for all: rebates or surcharges are possible, and make sense. I apply a sliding price scale to my work. Those better able to pay, pay a premium, and thereby subsidize students (or activities) less able to pay but no less worthy.