For a typical scheduled longhaul pax service, say between Europe and the Far East, the passenger yield is about 6 or 7 Euro cents per kilometre. The freight yield per tonne is about 17 cents per km. So a tonne of passengers - say 11 of them including their bags - generates on average 4 times as much revenue as freight.
But as others have said, some freight is high yielding and may take precedence over passengers. Since freight travels one way and flows may be highly directional, aircraft may be carrying high-yielding traffic one way and filling up with whatever they can find, at little better than marginal prices, in the other direction.
When calculating cargo capacity you also must take into account the density - aircraft are limited by weight as well as by volume, You can fill an aircraft up to its max weight with gold bars and there will still be plenty of space remaining. For more information I suggest you ask the Freight Dogs.