That's why the cost is the square of the number of votes. Then someone with 100x the wealth would be able to only buy 10x the votes. If you consider the act of lobbying for policy to be equivalent to purchasing votes then currently the cost is linear. Of course by that analogy there's no real way to enforce quadratic cost on lobbyists, but perhaps if there was an alternate "legitimate" means for individuals to purchase voting power then they might follow such a scheme if the alternative were illegal and strictly policed.