Personally, I avoid falling into the trap of upgrading holdings the instant I have enough money to. Of course you're going to feel like you never have money if you're always spending it. I set a "saving point" based on how large my realm is. At the very start, it's around 500 gold. That means I will never upgrade a holding if it takes me below that amount. Why is this useful? Because it pretty much means I always have cash on hand if an opportunity arises to spend it. A nearby realm is breaking up and I need some mercenaries to grab a quick province? The gold is available, if I needed to save up the opportunity might be lost. I need to sway a fence-sitter into my plot? I have bribe money. Don't spend yourself down to zero unless you are getting an immediate return. A holding upgrade is a long term payoff and you shouldn't be spending everything you have on it.