No offense intended, but simply reading this left me feeling 'throw in the towel' exhausted.
That doesn't mean it's a bad idea at all, actually -- for some games, this could be very helpful, particularly if it only kicks in after a certain point.•
Back to the exhaustion for a moment: some people enjoy sheet maintenance, min-maxing, and any or all other forms of sheet and mechanics math. It's possible these folks would love this, too. There is also a non-trivial portion of the playerbase of any given game that does not, not for a hot second, want to have to math in their pretendy fun time games. I speak for no one but myself in saying I would enjoy this roughly as much as I enjoy doing my actual taxes, and to drive home the gravity of this statement, I say this as a self-employed artist who has to do the long form every time and has to pay up since nobody does any handy-dandy withholding for me that might mean going through all of that hassle means I get some kind of refund at the end of it.
Mechanically speaking, however, this is going to depend strongly on the system. The 'tax' would be one thing under a GMC/CoD setup with flat costs, and something entirely different in a system that uses multiplied costs such as nWoD or oWoD.
Even under GMC/CoD, you're going to end up taxing some character types more than others. One dot of <Powerstat>, for instance, will cost a player the same as one dot of Contacts (Bloggers) -- never mind the fact that these two things have nowhere near close to the same impact on the game. It'd end up making it more effective for players to pile more XP into high cost powers, rather than low cost merits. When you look at the fact that the latter are the only thing available to most minor templates, it definitely gives the minor templates the crap end of the stick even more, as their powers tend to be bought on merits -- so the thing a super could do with one dot of a discipline will be something that costs four dots to a minor template as a supernatural merit, thus costing the character who is already 'the little guy' in all other respects considerably more simply to keep what s/he has.
It's an interesting thought, and the inspiration is understandable, but I wouldn't go within a mile of a game that implemented this as described, as I don't consider it to be something that actually improves the game experience in any way, and instead penalizes certain character choices and does so unevenly -- in addition to being more math than I feel like I need in my life.
• You would be better off doing something similar to what a few other games have been described as having done -- diminish the amount 'a beat' is worth as the character's XP total gets higher, and ascribe the 'lost' XP to maintenance. That hits across the board evenly regardless of what style of character the player has chosen to create. For instance, at 60XP, a beat is now worth 0.175 instead of 0.2, at 90XP, it's worth 0.15, at 120 it's worth 0.125 -- or whatever other staggered reduction metric you prefer.