Basically - mercs from real players would be like this - renting them. You can't afford to purchase that many for yourself, but you can hire them. If you are victorious in this big battle, you will have enough money to pay for the mercs, and have a lot left over. However, if you lose... well, that is how nations rack of debt. You would end up with little army, and a big debt to the player to pay for losing his troops.
It is like renting. You pay to get the thing for a certain amount of time, but you pay again if you lose it.
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