It's not necessary for a single wealthy individual to be able to buy a victory by themselves for the results to be skewed towards the will of the wealthy.
If 1000 votes cost £1 million, and 100 wealthy individuals all decide to spend that much in an election, that's 100,000 votes when these individuals would only usually have 100 votes. That's going to have an impact, especially on local elections.
As for it being more cost effective to buy votes the usual way, the "usual" way relies on voter fraud and massive advertising campaigns, one of which is illegal, and the other is very expensive and the voting decisions are not guaranteed. On the other hand, quadratic voting would be legal and guaranteed.
If 1000 votes cost £1 million, and 100 wealthy individuals all decide to spend that much in an election, that's 100,000 votes when these individuals would only usually have 100 votes. That's going to have an impact, especially on local elections.
As for it being more cost effective to buy votes the usual way, the "usual" way relies on voter fraud and massive advertising campaigns, one of which is illegal, and the other is very expensive and the voting decisions are not guaranteed. On the other hand, quadratic voting would be legal and guaranteed.