The only way to make the sport cheaper is to have only 1 manufacturer. Then they can spend the minimum and still win.
As soon as you have multiple, the cost will always be what they think the value of winning is. So as long as toyota and Hyundai want to spend 100 million or so per season to be able to day they are wrc champions, the others will have to do the same to keep up.
Even if you would downsize to rally2 cars, they will still spend roughly the same amount if that is what it takes to win. But while doing so they will also make rally2 way more expensive than it is today if you want to be at the front.

It's all down to how much marketing value a sport has, and nothing else. That why nobody cares about football when women are playing.