im trying to learn 3d sculpting (for hobby) but the z-brush software is like super expensive for an hobby. im not sure it has cheaper version for students like 3ds max?

and once i purchase the permanent license, the update for latest version will be provided permanently as well? or do i have to re-buy it whenever newer version come out

As far as I know, they don't have a student version, but they do have a Core version available that's much more affordable for students.

I know that there's Sculptris which is a free program. I believe it's made by the same people that make ZBrush. It's very basic in it's tools and options, but I think it might be enough if you just want to get familiar with 3D sculpting.