I know I have walked in to an Apple store and purchased an UPGRADE disc for $29. I will never forget this because when I later decided to wipe my hard drive and reinstall the OS, I was stuck because it was an upgrade only disc. I believe it was a Tiger upgrade disc.
More recently, my boss went in to an Apple store and bought a $29 Leopard upgrade DVD that I have at home. I can not use this disc to do a full installation.
So as I understand it, $29 gets you an upgrade disc and I would assume the standard FULL installation disc is $129. Apple's marketing department is putting a spin on this to full the press and it's customers.
I posted this question on the Apple board and got my question removed due to rumor and speculation! Considering Pogue's recent article, I think the "rumor" of the $29 Apple OS X installation DVD needs to be put to rest. There will be a lot of unhappy customers trying to do full installs with an upgrade DVD.