Historically, the discounted price for the upgrades have been reasonable.

If, however, you want any of the new features present in the coming year's version, you have to pay got get that new - stand alone - version. It is, after all, a perpetual license only with a revenue cap. The way the Steam versions have consistently worked is that you always get to keep the version that you buy that year. Honestly? I wouldn't be too shocked if they did to Substance what they did to Photoshop and Premiere and just released a stripped down Substance Elements type package.

v2022 seems likely seeing as how they've not given us any notice that v2021 will be the last such release, but v2023 is anybody's guess. Now that the Substance package has officially been incorporated into Adobe's Creative Cloud, which was long said wasn't going to happen, there are no real guarantees regarding future Substance indie versions. I will say, however, that you should probably jump at the upgrade at whatever price they offer.

Adobe creative cloud substance painter