We did it last summer and my daughter had a blast!!! She enjoyed it so much that she made a second one for her Papa a different day.
I believe the cost was between $15 and $20 and included the t-shirt. I don't believe you can bring your own shirts as they need to be tied in a specific manner to get the Mickey to appear but I'll be honest and say I didn't ask either. They have an abundance of shirts in various children and adult sizes already tied up and ready to go. I felt the price was reasonable for the shirt, dye, teaching, etc.
Both children and adults can participate.
Just a heads up that once the shirt is finished they will wrap it very tightly in plastic wrap and put it in a plastic bag. It is recommended that you leave it this way for 3-5 days I believe. They will also give you washing instructions.
This is a great Pop activity and one I highly recommend to others!