Is the amount you pay for the upgrade calculated from the cost of your room or from the amount that you paid for your room?
For example I booked a 11B for 3adults, it should have been 2400 but because of the special deal for Canadians I paid 2000. So would the cost of upgrade be calculated from 2000 or from 2400.
I, too, have been researching this and it seems the price difference is based on the selling cost of your current room vs. the selling cost of the room you want to upgrade to. So, that means a few things:
1) Sale price or discount is not factored in. The "starting point" cost is whatever they would sell your room for that day. So, in that sense, it's an even better deal if you did not pay list price for your room.
2) Number of people in the room doesn't matter. A couple upgrading from an 11B to a 5A will pay the same upgrade rate as a family of four doing the same upgrade. (Or so I have read here). Therefore, it is an even better deal for families/large parties.
3) If you booked IGT/OGT/VGT, it gets a little more complicated. I think I read here that the starting price is based on the lowest room you could have received, not the one you actually received. (Or maybe it's the best room you could have received, now I'm confusing myself!)