It's a very broad subject, so you'll want to define the scope a little more. Biggest bang for your buck is probably to go into learning VBA and the programming portions. A good programmer can make anywhere between $100-400. Statistics could also be a good course to take, as it lets you understand functions. Need to understand logic trees, and how they work. Financial Classes and financial analysis "could" help you udnerstand what your customers want.
As I said at beginning, you need to figure out what do you want to do with XL, and that will help narrow down what you need to learn about.