The enhancement plan costs 16%, the maths on this says that you are going to pay for the software again in just over 6 years
So in order to save 25% you should wait 8 years then buy the complete system again
Considering MS is releasing a version or new release every year, this means that you will be 8 version out of date and if you look at the improvements in the system this just doesn't make sense
I find most of our clients are upgrading every 2-4 years, however we do application upgrades every major version where we can to every client (2013 being the exception here)
Bottom line, paying the 16% is worth it
Cheers
NEv