I would disagree. I think the system should be based on the number of voters that actually support the third party. One person should be allowed to spend only a very small amount. A group with thousands, tens of thousands or hundreds of thousands of members should be able to spend a multiple of that.
Businesses are legally one person. Business executives are very few in number and don't represent shareholders or employees politically, so businesses should be very strictly limited in what they can do. Citizen groups that are supported by tens of thousands of members—many are supported by many more voters than parties are—should be allowed to spend a multiple amount of what one voter could spend.
That would be an egalitarian system that fits with the Supreme Court of Canada's egalitarian model.