By "at cost" do you mean they're paying your mortgages and property taxes for you? If so, they could afford to buy if they had a down-payment. They probably don't have a down-payment because all their money goes to rent :)

I don't blame people for being capitalist when living in a capitalist system, but it still sucks. You could try something like a non-predatory form of rent-to-own where they gain equity over time (though these arrangements are usually predatory).