On average, it's 12% to 18% overall. In specific regions where, say, 80% of all construction companies are union free, the cost goes even higher. In an area where you have 60% to 40% union free versus union, the difference is a little less. In the province of Alberta, where close to 90% of construction is open shop, when you have union-only requirements it's even more expensive because the pool of workers and contractors you have to go with is so small.
On April 30th, 2013. See this statement in context.