The longer your visit, the better the deal. If you stay for 3 days, you'll be spending about $7/day on coffee, tea, soda, etc. If you stay for 5 days, it comes to about $4/day.
You can't use the mugs for refills at the theme parks. So, how much time will you be spending at your resort? (For instance: We love having the mugs for morning coffee, while eating resort meals, and sitting at the pool.)
You can figure on a $3.50 – $4.00 charge for a soft drink or coffee purchase at the resort. This adds up fast if you enjoy multiple drinks per day, and/or have a long visit planned.