Hi Keith
Are you on Subscription?
If yes, that would explain why you've received them free of charge with each release.
As far as I'm aware if you only bought (No Subscription) -
AutoCAD R14
AutoCAD 2000i
AutoCAD 2002
you had to pay an additional amount to get Express Tools.
As far as I'm aware if you only bought (No Subscription) -
AutoCAD 2000
you could download for free the Express Tools from the Autodesk web site (obviously Autodesk had a moment of weakness during that period).
If you only buy (No Subscription) -
AutoCAD 2004
AutoCAD 2005
the Express Tools come "free" on the installation CD. Again Autodesk are showing their caring, giving side
Have a good one, Mike