There are two approaches that come to mind. (There are probably more.)
1. Hirelings are "monsters" who act as normal monsters do.
It's easy to price out the cost by level in this case: 5 monsters of Level X is one Encounter of Level X; an Encounter of Level X is worth a single Level X Treasure Parcel; therefore 1 monster of Level X = 1/5th of a Level X Treasure Parcel.
I use "Random Monetary Treasure Parcels" (or RMTP) to determine the value; roll 1d6+4 on the appropriate Treasure Parcel table to get a result. I like the random amount, I find it adds a little something to the game. For example, in one game when PCs hired a group of linkboys, I rolled the minimum amount on the table; I decided on the spot that it was because they were addicts out of cash looking for a fix.
2. Hirelings as special abilities the PCs can make use of.
In this case I'd give each hireling - or group of hirelings - a Power that can be triggered by a PC. The PC would have to use an Action to trigger the Power, though that might not always matter. You'd look at comparable magic items to see where such abilities lie.
e.g. Lockbust Chalk allows you to make a +7 Thievery check against a lock for 480 GP. Translate that to a Navigator/Pilot, who, for the price of 480 GP will make +7 Nature checks for you. I'd assume that once you pay that price you have the guy for life, and handwave payments/salary that aren't relevant to the game's economy.
I could go more into this but maybe later.