The cost of becoming a master gardener will depend on where you take the course. The Lincoln county master gardening program (http://extension.oregonstate.edu/lincoln/master_gardeners#what is MG program) costs $195 while at the University of Florida, the cost of the program will depend on the year that one joins the program in.
Knowledge is power. You can learn on your own and find out by trial and error or you can go to school to be more advanced. If you chose schooling to become a horticulturist it depends on the school you go to.