Typically, the price offered is less than the combined value if you were to straight buy them in Europe: 2000 for soldiers, variable artillery price. I've gotten some great deals though, like 4 veteran soldiers and dragoons for ~4000.
However, the foreign mercenaries you get turn into veteran soldiers and dragoons (or Continentals in the Revolution). Why you would you want the enemy on your side? They should be slaughtered!