They would be subject to the same limits as a third party wishing to do advertising during an election campaign. It's based on the province, and the limit is adjusted for the province as a whole. It's based on the voters, the area and the number of places involved. There is only one area, and therefore, there is only one nominee. The limit is set using the formula laid out in the Canada Elections Act.
On April 9th, 2008. See this statement in context.