For myself:

• I bought a computer in 2001 that came with MacOS 9.2 and Mac OS X 10.0: running total of $0 on OS
• Users of OS X got OS X 10.1 for a media fee: running total of $15 on OS
• I bought OS X 10.2 for $80 educational (was $130 non-educational): running total of $95 on OS
• I skipped OS X 10.3 because I didn't like it, effectively saving $130.: still running total of $95 on OS
• I bought OS X 10.4 for $130: running total of $225 on OS
• I bought a new computer in 2007. Had I waited a bit I could have gotten OS X 10.5 for free. Paid $130 instead: running total of $355 on OS
• Am buying OS X 10.6 for the $30 or whatever: running total of $385 on OS.

Likely this version of the OS will last 2 years, so that's less than $400 for one decade and two computers.

um.... didn't they? I have folders of folders of playlists. and some of those playlists are actually folders of several playlists.