You don't mention how many days are on your passes. This is how the pricing works when you upgrade at the gate...
Extra cost on top of the base-price ticket is a flat rate of $50 ($53.25 with tax) for add-on to a 1 day Magic Kingdom ticket; $58 ($61.77 with tax) for add-on to a 1 day Epcot, Disney’s Hollywood Studios or Animal Kingdom ticket; $50 ($53.25 with tax) for add-on to 2 or 3 day tickets; or $64 ($68.16 with tax) for add-on to any 4-day or longer ticket. This works out to a little over $9 per day on a seven-day ticket.
Upgrading at the park is a set price for any 4 day or longer, where it looks like UT doesn't offer that benefit?