The average journal profit margin is 20-30% and that's roughly the same whether you're talking about an open access journal like PLoS or a closed access one. They also have roughly the same operational (staff, etc.) costs. Most new journals tend to be run at a loss for a number of years being subsidised by the more successful ones.
Marketing is another big cost, journals don't magically build reputations, they have marketing budgets to achieve mindshare and to establish a reputation.
There are good reasons for open access, but lets not pretend that journals are cheaper to operate than they actually are.