Only Europeans can purchase mercenaries in Europe at high cost (20d per infantry, 40d per cavalry, 65d per artillery). Mercenaries will always cost more than if you were to recruit the troops yourself (max base to recruit: 16d per infantry, 35d per cavalry, 40d per artillery). For rich countries that are fully naval, full quality, fully free subjects, paying 20d per 1k infantry compared to 16d to recruit might not be that bad of a deal. Recruiting troops causes war exhaustion and one needs to pay small amount of maintenance for troops during peace. However, there is a limited pool of mercenaries that increases each month and they can only be hired in Europe. Provinces of Asia Minor and some few provinces east of the Volga River count as European provinces. As time and tech levels progress, artillery components are added to the mercenary units, making them even more expensive. However, paying 65d per artillery compared to 20-40d is a rather bad deal. Unfortunately, one cannot change amount of troops of each type as the deal is a package.
If you are low on manpower, mercenaries can be a life saver for rich countries (in MP especially). Otherwise, the more you can afford mercenaries, the less you need them; still, they are a valuable resource that can get you out of a tough spot. There is a kamikaze strategy for minors that calls on asking for several loans, recruiting all the manpower pool, and hiring mercenaries to beat your enemies before going bankrupt. Use this strategy at your own risk as bankruptcy lowers troop morale for five years and nets you 10+ inflation. Another strategy when you are quite rich is disbanding your armies to get a DoW and then hiring mercenaries to start fighting while the new armies are trained.