Hi Meg,
It depends on the type of ticket you purchased.
MyMagic tickets are purchased for a specific length of time. With that length of time, the options, should you choose them, correspond to the same length of time. For example, a four day MyMagic ticket would have four days of park hopping or four options if those were purchased as an extra with the ticket.
You are correct that you only receive a DisneyQuest extra if you purchase the Water Park Fun & More option which would give you access to the two water parks, DisneyQuest, ESPN WIde World of Sports, miniature golf and the Oak Trail Golf Course. You can use those options in any combination. So going back to our example above of a four day with Water Park Fun & More, you could do four days at the water parks or two days at a water park and two days at DisneyQuest. The combinations are many but do not exceed the number of days you purchased.
Currently, prices for adults 9+ is about $50 and children 3-9 is about $43.
You can always confirm your ticket options at the concierge desk at your Walt Disney World Resort or at Guest Relations at any of the four main theme parks.
Wishing you a magical vacation!
Beth