Merc maintenance is 2.5 times more than base unit. So basically, a merc inf maintenance is that of a canon, and more than regular cav. Saying it's cheap is quite wrong. Overall, if you wan tot play with merc and aren't filthy richy, in which case you won't care, you should keep a decent a mount of inf around and all the cav/canons you could want, and be under FL in peace time. During war time, fill in extra spots with mercs and then go ahead. Overall though, unless going crazy with wars, I'd say merc spam should be the solution when you're desperate. Alliances in themselves are enough to protect against major threats most of the time and merc spam is a good way to destroy your economy. The AI might just wage one war every 50 years, so it has time to recover. You probably won't want to do it that slowly. I would only advise occasionnaly a couple mercs with your regular infantry when in need of extra manpower, or to siege high attrition places (the wiki is lying about merc and WE btw).
