Go Pro! Money for Roll20 means more awesome crap and considering how much time you spend on here, a few bucks is worth it.
Though my observation having run and played a few games, is that automating substraction of traits is actually really hard to do, not because of API calls, but because there are so many exceptions to certain rules. Each rule you try to add or subtract to an attribute of any kind adds a non-reducable level of complexity that can be hard to manage. At that point it's less like a tabletop rpg, and more like a video game.
If it's a quality of life improvement for players, i say make them track it. If it's to make your life easy, then you could have 'henchmen' npc's that have weak abilities that they always use and just ignore costs, and then have badass npc's that you actually track. I'm all for limiting cognitive overhead and letting computers do math, but it's tough to have a press a button ability macro that subtracts values from another thing without slowing the game down trying to troubleshoot macros, or undo ability button presses people did during testing for example. D@D is fundementally a game about communication. Anything making that fast is good, anything slowing that down is bad, and i see alot of badside in trying to force the system to track simple math without human input.