For supplies - I am willing to buy the book or pattern for the class -
If the class is to learn how to buy a special/unique tool or ruler - I might be willing to buy it - but would want to see it demonstrated first.
For fabric - I have a "bit" at home - I really prefer to use what I have, if possible.
If the shop/class makes it clear that it is okay to use what - especially fabrics - I already have - I would be more willing to sign up for a class. It is highly likely that I will "need" something while attending the class - so I probably would buy whatever at the shop then.