While taking a PDC may certainly be the fast track to learning about it, here are some alternative suggestions: FIRST
: If you do nothing else, see the thread below
here in the forums. It's a link to 38 recorded class lectures and field trips from a permaculture design course offered at North Carolina State University. It's like sitting in on a college course for free and without having to do the quizzes or homework. It's packed with very great information and I hope you will find it a fantastic resource to get you started. http://www.permies.com/bb/index.php?topic=6324.0
: (looks like one person suggested this already but) Check to see if there are permaculture courses offered at a local community college in your area. I was lucky enough to find that the community college in the next city over from me offers a permaculture design course so I signed up and am currently taking it. The great thing about it is that the cost of enrollment and registration at my community college was FAR below what a usual PDC would cost. Third
: I'm not sure what books you've been reading already but check out "Gaia's Garden" by Toby Hemenway, and/or "Introduction To Permaculture" by Bill Mollison, both of which are pretty affordable and great introductory resources which I'm currently reading. And there are plenty more out there.
If you want to take a PDC because you want that certificate to be able to legitimately call what you do "permaculture" then yes, you'll have to take a course, but if what's really important to you is learning, there are plenty of other ways to learn permaculture without taking a course so don't be discouraged. It will be work, but you can do it!
Oh one more thing: Prepare for Failure. You will have some, but that doesn't have to discourage you. Plants will die, diseases and pests will cause problems, and there are plenty of mistakes to be made on your journey. Don't let it get you down, just accept it as a natural part of your journey and learn from it when it happens. One of the quotes my permaculture professor put up on the board one day was "If you're not making mistakes, you're not pushing the edge of your learning." And edges are an important part of permaculture so push the heck out of those babies
I hope that helps!