I think in general the 10% figure is thrown out and to me that would be the bottom.
There are definitely cases where it could be much higher than 10%, and it depends on a lot of things.
One way to think of it is, think of Canada as another state like California in population, where .ca is highly recognized and trusted, and maybe most importantly there is high internet penetration, high rate on online shopping and use, good income levels, and an internet savvy population. As a market for an online venture, Canada is pretty top of the line, majority English speaking, and right next door to the US which makes logistics easier for US companies and imports/exports. If you are a big enough company maybe the .com is enough, but as well you probably want to protect that brand in a .ca if you have significant operations in Canada, or plan to.
Often though it is hard to compare .com vs .ca prices, because the .com has been owned forever by a company, or it sold 10 years ago so the price is outdated.
Voice.com sold recently for $30 million, does that make Voice.ca worth $3 million US?
I have an example in past year where I was offered 2k range by a broker for a .ca, then did some research. Broker then asked what price do you want, and I just replied I'd rather deal with the decision maker and not a broker in this case. Later I saw the .com version of the same domain sold for mid xx,xxx range also in past year. Given I know the company operates in US and Canada, I would not be asking %10 of the .com price (mid x,xxx). In this case I'd probably be looking for at least $20k, so that would be around 40% of the .com sale. Maybe even a little more, as the name has a couple more potential uses.