The citizens of your kingdom and their lives and livelihoods and homes and farms and city buildings are both the OUTCOME of BPs, and also the MEANS by which BPs exist. They *are* the resource you are spending. Just like when you roll up a new character and "spend" your starting money, you aren't going down to the corner store with a sack of loot to buy a suit of armor, a sword, and a backpack. Your "starting money" represents all of the wealth and resources you have accumulated in your life prior to starting your career. In your backstory, your sword was your father's old nicked blade from the Goblin War. In pure game mechanics, you "spent" 20 gp on it, but all that represents is that "20 gp worth of your game mechanical wealth is represented in the form of this sword you possess," not that you literally spent 20 golden coins to get a shrink-wrapped blade at Ye Olde Weapyne Stoere. BPs are an abstract means of assigning a value to what the PCs control in a game-mechanical sense. Your kingdom encompasses a certain amount of material and immaterial wealth, the vast majority of which is in the form of stuff that your PC's don't actually OWN. BUT, while they don't own it, they as the rulers of this new land, as long as they maintain the goodwill of the people, do get to control it - not on a micromanagement level, but they get to decide, in broad brushstrokes, what the citizenry of the kingdom will do, and where, and when. Found a city, build roads from here to there, plant farms, expand commerce and business opportunities.
I think if you look at BP expenditures as what the leadership of your rulers CAUSE to happen in the kingdom (in month-long increments, remember), rather than a personalized kind of traction the PCs are doing out of their royal checkbook, I think it makes a lot more sense.