mercs cost half as much to hire in comparison to when you would train (if it was possible) the unit, BUT they cost double in upkeep.

There are a lot of threads about mercs but the main idea about them is they are there to use in a pich, to make your crusades/jihads bigger (when in crusade/jihad no upkeep must be paid) or to use in armies that can't normally produce such units (e.g. horse archers in european armies or longbows for any faction that isn't England).