Adults cannot dress up as Disney characters (ex. Elsa, Cinderella, Aladdin, etc) because guests may mistake them for Disney cast members. The only exception is during Mickey's not so scary Halloween party events.
Their price changes depending on the demand/season. Check their website for current prices.
Price for adults
The 1-day prices vary from time to time, and park to park, and also increase every year. Be prepared to spend well over $100 for a 1 park 1 day pass. However, there are discounts and special prices if you get more than 1 day, or if you are buying a for a large group of people.
If you mean what is the Disney Parks definition of a costume that would be not allowed in the parks, I have been told by an employee they are mainly referring to masks that cover the face. I think the costume policy exists for public safety issues.
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