Not sure I've heard of a model exactly like that before, but it sounds fairly similar to "1-year updates + fallback perpetual" which is the JetBrains model. I'm kind of leaning towards that, with a monthly option for a lower commitment:

- Monthly. No perpetual license.

- Annual. Perpetual license for the latest version as of your subscription renewal (a little odd, but matches JetBrains).

Both in the range of $5-12/mo total. I could also consider free/discounted plans for educational and non-commercial open-source development.