I think I understand what you're getting at here, but I think this is over the top.
As an instructor I'll bet that I've spent more on training than 99% of the people here. Through my instructor training and classes I've taken just as a student, I have spent more than many have on their guns. Even so, what I've spent on training doesn't come close to the dollar figure I've spent on guns alone.
Besides, the numbers aren't really equal. An M&P is a fine defensive handgun. They are available for less than $600. Buy two and you've spent $1,200. A trip to a prestigious gun school will cost around $2K so, according to your formula, that's the right amount of training. What if I buy a Wilson Combat? Now I only have one gun and that trip to the same school is not enough now because it doesn't cost more than the gun? See, it doesn't track.
I think training should be recurring, just like practice. One trip to any school doesn't make anyone an expert. A training class a year would be great, but not everyone can do that.
I, like most here, think that training shouldn't be a mandatory prerequisite to owning a gun. However, a conscientious gun owner, who wants their gun for defense, will seek training rather than try to justify not having it.