I'm more concerned about your statement "I think it's excessive as designers are not paid fairly anyway." This sounds more like you need business advice rather than Archiplus advice. Can you raise your prices by 5% to cover the software costs? Can you take on one small project to pay for your primary tool? If you bill hourly, can you move to fixed pricing for portions of the work? (I hate working hourly. As I get more and more efficient with Archicad, I lose more and more money unless I raise my hourly price.) Can you use remote drafting with a markup?
It is true that some version upgrades are more valuable than others. There is usually one or two features per upgrade, though, that save enough time to pay for the upgrade. (But again, not if you bill hourly.) But you won't know which those are in advance. Overall, I think you are better off with Archiplus, but you need to have the "big picture" to feel okay about it.
Richard Morrison, Architect-Interior Designer
AC25 (since AC6.0), Win10