Based on their publicly published prices, I found
UT to be about a 4.8% to 7.3% discount.
Examples (taxes included):
5 day tickets for 3 children and 3 adults look like this:
Disney: $1,543.19
UT: $1,459.01 a savings of 5.45%
5 day tickets for 3 children and 3 adults with Hopper:
Disney: $1,894.64
UT: $1,803.71 a savings of 4.8%.
Note: The discount increases to 7.3% for a 6 day with hopper... because there is no 5-day with hopper (5 day ticket comes with a bonus 'free' day, so 5 and 6 day hopper tickets cost the same).