Various things can be purchased for a kingdom in order to strengthen and modify the defenses and quality of life for those living there.
When a kingdom doesn't have enough troops to defend itself, it can outsource mercenaries to do their bidding and fight in their place. Mercenary companies' prices are determined largely by their quality and quantity, with larger and more well-equipped groups being expensive while small rag-tag bands are cheaper and can be purchased en-mass.
Mercenary companies cost 2 Wealth per Troop with an upkeep of 1 Wealth per Troop.
Bonus Troops such as those from Kingdom Traits or Special Building Troops have no cost, only upkeep.
Buildings, outside of character creation, are purchased using wealth as opposed to INF; increasing the population, defenses, or other factors.
Resources may be used to reduce build time down to a minimum of 1 turn, this is for buildings and settlements. Each resource may only be used once per building or settlement, their effects stack linearly (1/2, 1/3, 1/4).
Wealth equal to the building cost may be spent on your turn to reduce the remaining build time by half.
Prestige equal to half the building cost (Minimum 10) may be spent on your turn to reduce the remaining build time by half.
Piety equal to one quarter the building cost (Minimum 5) may be spent on your turn to reduce the remaining build time by half.