Penn-Foster, in my opinion, is not worth the money. It is a diploma mill (or "certificate" mill).
First off, in the way of full disclosure, I will tell you I did not learn/study AutoCAD by way of Penn-Foster although as Cad64 so graciously mentioned, I have helped dozens of their students with the projects Penn-Foster assigns as part of their coursework.
What is Penn-Foster charging for their AutoCAD based certification course now-a-days? Just curious.
The major complaints from students who have sought help here at CADTutor are 1) instructors who do not reply to their emailed questions, 2) instructors who lack specific knowledge of a discipline, 3) a tendency to point the student at CADTutor to get answers to their questions, and 4) project instructions that are erroneous, missing information or just plain wrong.
Why AutoCAD and not some other CAD program? What other avenues of instruction have you looked at for learning AutoCAD? Where do you live (United States, UK, other)? Do you have a specific discipline in mind such as architectural, structural, civil, mechanical, other?
BTW...I learned AutoCAD mainly on my own starting back in 1985 when courses were barely being offered by AutoCAD resellers and nowhere else. But over the years I took advantage of other methods to expand my knowledge.
In conclusion, I'd be happy to answer any other questions you may have on the subject.