This is a bad idea for the same reason BP-Chargen is a bad idea, it will heavily heavily favor min-maxing stats. Vanilla Karmagen makes humans more viable / common, just as they should be more common. You do this rule in Karmagen and there is absolutely no reason to ever be a Human, ever.
Metatypes already have their Metatype Cost added to the "maximum allowed points" to be spent on their Attributes under Karmagen, I'd strongly recommend leaving it with that.
It's a workable rule if there's an appropriate charge for the metatype, and all metatypes have the same attribute budget. And if you have to spend at least enough on you reduced cap attributes to account for the negative.
Dunno, I've crunched the numbers regarding what all the metatypes are worth if you start from a vanilla "no bonuses or detriments", getting this. I've factored positive and negative costs, including lower maximum attributes limits on metatypes like dwarves or trolls:
Humans - 45 Karma, 0 after discounting human cost to return them to a 0 baseline
Elves - 95 Karma, 50 after discounting human cost
Dwarves - 175 Karma, 130 after discounting human cost
Orks - 100 Karma, 55 after discounting human cost
Trolls - 122 Karma, 77 after discounting human cost
Obviously the costs don't line up like that in the books, so there's some sort of weighting going on that changes things. In the end, I'd prefer to pick metatype by backstory/concept and pay whatever cost is necessary.