It's not an easy thing to answer as CM's at Disney don't deal with it consistently. To get an idea go on Disney website and look at the price difference there (between hoppers and non-hoppers regular prices) That is the minimum amount Disney will charge to upgrade.
Now here's where it gets complicated. If the tickets you purchase are Canadian
discount tickets some CM's will charge you the difference between the value of the Canadian ticket and the value of the Hopper ticket (non Canadian discount).
You might get lucky and will only be charged the difference between a Canadian discount regular and Canadian discount hopper. This is the option I consider to be fair but doesn't happen often in reported experiences.
Upgrade (non-hopper to hopper) can only be done in person when you are in Florida. There have been many threads here on Canadian DIS which discuss this very issue. Some people get pixie dust some don't and that option can be very expensive.
Personally, when I have Airmiles non-hopper tickets we just don't hop. It can take a little more planning to make sure we get everything done but I've still had enjoyable holidays.