There's almost no cost. In Canada, we have extremely high snow loads. For example, a roof is typically built to hold up 50 pounds per square foot, or something like that. That's the amount of weight that is required. Solar panels are three to five PSF, so you're talking about, at most, a 10% increase, which would increase the cost very negligibly, especially when you're talking about truss roofs.
On February 15th, 2018. See this statement in context.