I would say the upgrade pricing is a good deal. Plus that gets you onto a supported version of Sonar. That could come in handy if you want to use the latest Windows, and Microsoft changes some code that breaks something in older application software. At the moment X1-X3 still works, but no more patches or updates for the Sonar X series!
I have several other software packages that used to work fine with Win XP, but are completely useless to me now on Windows 10. It was stuff that I only used on occasion, so couldn't see paying for upgrades. Maybe should have upgraded some while I was still eligible. Something to consider ...