The arms and equipment guide has some guidelines, but there's also a bit of information in goods and services. A trained hireling (teamster for example) requires 3 silver pieces per day. So you could probably get a guard to guard your warehouse for 3sp per day. A trained mercenary would probably require significantly more than that, as well as a hiring bonus, as well as a surcharge for battles. Also, he wouldn't want to be used as fodder, front line troops, or fight anything with an equal or higher CR to him. I'd say a 5g-10g signing bonus for a level 2 warrior (npc class) mercenary with decent armour and weapon, 1g per day, and at least 10g per battle. For cavalry, x2-x5 those costs, for a good archer at least x2, for heavy infantry or infantry specialized in a role (fighter classed, or warrior level 3+, monster hunters or a team of tunnel clearers/kobold killers) x2-x5, etc. For something like ogres or maybe a young dragon, you're looking at hundreds of gold, and a good chance of getting killed by them if they want the treasure you just looted.