Cost of creating an army (From the Mass Combat rules)

[Kol Korran]

Hi there folks. In a game I'm running, I want the PCs to get an allowance to form an army (Made of some smaller "armies"), using the Mass Combat rules. I'm new to them, and I may be missing something. They detail Building Points (From the Kingdom Building rules) used to buy resources and such. But I see nowhere the cost of acquiring an army. Are there any rules? Where? If not, has anyone made any kind of homebrew for this?

Thanks!