That is typical behavior for a lot of teens who are exploring their talents, interests and possibilities. In fact, most teens tell me that their biggest fear of the future is having to settle on one path (choose a college major, career, etc.).
I can understand how you don't want to invest a lot of money on something she'll drop in a few weeks. Make her a deal. You'll support her exploration of different activities but she'll have to help fund them. This way, if she has a financial investment, she'll be more careful in selecting an interest. Plus, anyone can tell you how teens will appreciate something more if they purchased it versus it being given to them.
Strike a bargain as to how much she'll chip in or what your financial limits will be for any given activity. Have her learn to earn her activities. Her true passion will surface when you see her investing her time, talents--and own money--into an activity.