I have taken all three Wilton cake decorating classes through Michaels. The first class will require that you spend the most money...I think I spent about $50 on supplies in addition to the class fees. They usually run specials for the classes. I took class I and III for $12.50 each (half price). It think our Michaels has new classes starting next week (July 21). You do have to buy a kit that they sell for each class, but you can use their 40% off coupons to do so (which I did). For the first class, you make a cake each week and take to class to practice on. DH and DD loved that. In the second class you learn to make all types of flowers, basket weave, use color flow and (I think) only make one cake at the end. The final cake for that class has been my favorite. I make a version of it quite often. In the third class you learn to use fondant (very easy) do some rope work, and put together a tiered cake. You do have to make one small cake for the fondant lesson too.
Other posters are right, you learn some wonderful techniques and can make beautiful cakes. I have made several special occassion cakes for friends (baby and wedding showers, graduations, birthday bashes). Several friends have suggested charging for my cakes, but I think that would take the fun out of it.
I hope you enjoy it as much as I did! If you have any other questions about the classes, I'd be happy to try to answer them. Also check out Wilton's website; they may have additional information about the classes.