I see you've discovered the real world dilemma that faced many warlords: how to make your military conquests financially viable without bankrupting your country. Realistically, you can't sustain a large military forever (at least, not without having your own society degrade drastically).
However, this is D&D so it probably comes down to how you convince your soldiers to follow you for long lengths of time. Humans would probably want to get back to their farms to plant/bring in crops. In D&D terms, I'd say who cares how much it costs. Do you really want to become a D&D accountant or economist? And if so, you'll discover how wanting the system is in this regard. I'd concentrate more on how you'll attract and retain your troops, rather than how much they cost. That has much more interesting role-playing and adventure opportunities. When it comes to money, just run a dragon slaying adventure and say the proceeds from the dragon's hoard pay for your army (for a time...).