I think the best solution is to require shares for any PC-class hirelings. The actual pay should be determined by NPC level and also by the party level. So if the 1st level party wants to hire a 5th level cleric he might not settle for just a share. He may even demand half a share unless they take the risks (and encounters) associated with 5th level adventuring. In which case they might drop like flies.
Since NPCs don't have any more level-meters than PCs, they could join up not knowing what to expect, become disillusioned and realize that he is overqualified, and then leave. If the PCs wants to keep him on despite the lack of loot they would need to either give him a much higher share (maybe 50% of the total share) or a substantial amount extra.
In the case of being an hired soldier/healer the pay would be per day regardless of there being combat or need of healing. The sum would be fairly high for a low-level party, but they would be able to expect obediance and whatever else was contracted.
Since the danger would be much less than normal adventuring, the cleric in question might not demand as much per day as he could expect to gain by normal adventuring, but still would be much higher than normal warrior mercenaries or priestly (civillian) wages.
While the PCs might get 3000 GP in a day they will not do so EVERY day. So getting 500 GP per day might not be too shabby, depending on what they are doing. An NPC hired to assist for a specific task will probably request a sum for the whole task and not ask for daily wages. They will expect the task to take a certain amount of time and will probably make sure they don't sign up for open-ended tasks. They will also generally ask for a large downpayment, maybe 50% before they even start. After all, there is no guarantee the PCs will pay anything.
In any case the sums involved will be campaign specific and not game system specific.
In my own game where 2 sp equals a daily wage for a poor laborer, a 5th level cleric can expect to get maybe 100 times that amount per day, depending on his station in life and willingness to adventure. It would be no guarantee though that the PCs would even be able to find such a person willing to adventure, as most NPCs of that calbre are either adventurers in their own party or high-ranking clergy without time or inclination to become mercenaries.
The common mercenaries will be 1st-3rd level warriors, and will require at least 5 sp per day for their service. They also would not be expected to fight against monsters, but rather other soldiers, humanoids etc. and usually not deep down in dangerous dungeons.
If the PCs wants them to do so they would either need to hire adventurers like themselves or get the Leadership Feat, in which case they could loose alot of Leadership Score by sending their minions to certain death.
