In the new Army rules in Pathfinder, all the costs for maintaining troops are reduced into a single Consumption score that includes wages, training, equipment, supplies, and so on. Also, for the sake of simple rules, any new troops you create become ready for deployment at the beginning of the next month and they simply cost their monthly consumption rate for the first month of service without any recruitment and training costs being accounted for as special "creation" costs.

Now in such a system, mercenaries would be really good. You don't have to wait until the beginning of the next month (since they are already trained, equiped, and ready for battle), they already come with battle experience (which comes in the form of a few special tricks the units know), and you only have to pay and supply them for the duration of the campaign.

Now based on historic accounts, how much more would the monthly "rent" for a unit of mercenaries be, compared to the monthly "maintainance costs" of having a standing army unit of comparable "combat power" (how much use they are in battle). Now, standing armies have not been that common during history, from what I know, but what would you say would be a good monthly cost for hiring a mercenary unit? Three times the cost for standing troops? Four times?