I've been through about 9 or 10 AutoCAD upgrades over the years, and that doesn't include every version ever released of course. Typically it was every about other version.
The cost of the software has never been the lion's share of the expense, even when seats were too "stale" to legally upgrade and/or we had to pay full price for new seats.
The majority of the expense was always in upgrading hardware, O/S's, installation, configuration, debugging, training, and dealing with clients who were either ahead or behind us in the upgrade process, fighting to maintain job schedules, etc.
In general I'd say the cost of AutoCAD itself (always plain vanilla in my case) was only 30% to 40% (at most) of the total upgrade costs; but this is certainly just a seat-of-the-pants figure as there were just too many specifics to track.
My feeling has always been that every other upgrade is probably sufficient to keep things from going totally sour. I certainly wish we'd skipped r11 & r13.
We managed pretty well going from r14 to 2000 to 2004 to 2007, but your mileage may vary. For instance we rarely do 3D, and while we work from other people's digital files we rarely do our work on them.