The final test of how your dreams of conquest will fill or drain your royal treasury is experience. In only a few turns the results of your actions will give you a better idea of how to budget. You will get an idea of how much wealth various activities bring in by looking at which of your actions were successful and which were not, as well as the overall gain or loss to your treasury. Remember that during war season turns you will spend more than you make. Each peace years turn you will make more than you spend. Deciding how much to spend may be more difficult your first turn, but becomes easier as you gain experience.

There some moves you can make that would be unwise at most any stage of the game, but would be disastrous at the start. These are:

1. DON'T concentrate your troops in too small an area. The cost of maintaining troops in a province increases exponentially with the number of troops you add. The quickest way to ruin the economy of your kingdom is to concentrate too many troops in the same province.
2. Never attempt to increase the size of your armies too rapidly. After all, somebody's got to stay and tend the fields. Your subjects will be unwilling to send off entire families to war in a distant land. At the outset of your conquest, a 10% increase per turn should be more than adequate, and, if you find in your first Kingdom Report that you will be levying large numbers of troops on a normal basis, you won't need to purchase many more.
3. Remember that different troop types vary in their expense. Depending on the amount of training, equipment, etc., they require, you may wish to balance your armies with some well equipped, expensive troops and some less expensive but more expendable troops.
4. If you want to pay attention to details, don't count every copper you spend. Rather, see how minor sources of income can keep your treasury from going dry. Cover your bets. Raiding, adventuring, and various acts of intrigue can bring in enough income to offset the possibility of a less than successful invasion.