The Resources we get from Shipments are fairly small or rather insignificant 

Current one oil shipment worth 1 or 2 diamonds which cost 20 crowns give only max 200 oil

Food Gold we get is around 5000-12000 for 9 Cider or Metal

There should be significant change in the quantities received as per ages

At or after Enlightenment  age the rewards should 1% to 2 % of the total capacity making it a wise decision to buy