Wow I think you probably got some major pixie dust in that deal last year. The value to go from day 6 to day 7 when purchasing regular Canadian tickets is around $10 US. Halloween tickets are in the $80 range. Even if you paid $10 more you are getting a real deal. I think last year was a very special case. I wouldn't expect it again this year.
If you wanted to calculate the upgrade cost it would be the value of the 7th day (Canadian 7 day ticket - Canadain 6 day ticket). Subtract this amount from the cost of the ticket you want, as Halloween ticket prices vary.