X-plane 11 is not final, it will be final in several years, but every revision in included when you buy the x-plane 11 licence.
At the moment, x-plane 11.02r2 is stable.
X-plane 11.05r2 might be the next stable release in a matter of days.
The first x-plane 11.10 beta should be available soon, with new objects and other art assets, plus more optimisation in terms of multicore use.
More information here :
There is no discount when you upgrade from x-plane 10 to x-plane 11.