This is sort of covered on p.16 under Mercenary Pay, but you have to extrapolate a bit.
Native soldiers cost 8 silvers a day to maintain, they recieve 4 of these as salary, the remainder going towards food, supplies, equipment maintenance etc. Those serving in the Free Companies are generally paid 7 silvers a day (not including loot shares) and are considered more expensive to keep than a native soldier. You could say that each member of a Free Company costs 14 silvers a day to maintain, based on a 50/50 split like a Native soldier.
The same section suggests that 90% or more of all fees that a Company charges their employer are used up on this maintenance. You can slide the scale a bit, but assuming 90% as a base, you could say that the basic fee charged per day for a Free Company mercenary is 15 silvers a day.
A Free Company with 100 men would ideally like to charge 1500 silvers per day to deploy their entire unit, 1400 to pay and maintain the unit, 100 profit. Note, however, that the rules for Locating A Contract on p.63 will affect this figure in practice:
LOOT ONLY:
Running Costs per Mercenary - 7 silvers per day
Salary per Mercenary - None
Standard Fee to Employer - 7 silvers per mercenary per day.
Profit to Company Commander/Owner - None.
ONE HALF:
Running Costs per Mercenary - 7 silvers per day
Salary per Mercenary - 3 silvers per day
Standard Fee to Employer - 11 silvers per mercenary per day.
Profit to Company Commander/Owner - 1 silver per day.
NORMAL:
Running Costs per Mercenary - 7 silvers per day
Salary per Mercenary - 7 silvers per day
Standard Fee to Employer - 15 silvers per mercenary per day.
Profit to Company Commander/Owner - 1 silver per day.
HALF AGAIN:
Running Costs per Mercenary - 7 silvers per day
Salary per Mercenary - 10 silvers per day
Standard Fee to Employer - 18 silvers per mercenary per day.
Profit to Company Commander/Owner - 1 silvers per day.
DOUBLE:
Running Costs per Mercenary - 7 silvers per day
Salary per Mercenary - 14 silvers per day
Standard Fee to Employer - 23 silvers per mercenary per day.
Profit to Company Commander/Owner - 2 silvers per day.
The numbers seem to work on paper, although I haven't actually put them into practice in the game yet. What do you think?
(EDIT: Changed 50 men to 100 men to keep the numbers in line with a standard unit).