This idea is pretty simple: when using the hotkeys to buy 10/25/100 levels at a time, if I have insufficient gold to buy that many levels, it would be nice if the tooltip showed how much I would need to buy them (as opposed to only seeing info about the number of levels I can currently afford).

Even better would be an option to 'lock' those hotkeys so it will only allow me to buy levels at all if I can afford the full number I'm trying to buy at once - after the very early stages of a new run, I only ever care about upgrading in multiples of 25, and it's annoying when I accidentally spend all my cash on less than 25 levels for one crusader (gaining a negligible amount of DPS) when I could have afforded a full 25 on another one instead. Alternatively, having the number of levels I'm able to buy displayed in a different color when it's less than the multiple I'm trying to buy would make these mistaken purchases less common (yellow seems like a good choice, pretty close to white so players won't mistake it for red 'you can't afford any levels' text, but different enough that it should stand out, at least to players who are looking out for it).

Simply knowing how far off I am from being able to afford 25 would be much appreciated, though. I'm imagining one extra line of text in the tooltip above or below the current info, to the effect of "Cost for [10/25/100]: ###."

Solistus, you probably started playing quite recently, so let me give you a piece of advice: leveling up all crusaders is useless, pick one crusader to be your main DPSer, and use every other slot to buff him. For example, it makes no sense to level up Sasha past level 5 (FIVE!) to get her Bulwark skill, at least until you need to reset. It's much more cost effective this way, and it solves your problem: level up buffers enough to get their buffs, then spend gold ONLY on your main DPSer.