Historically, upgrades have cost $79.99 US. If you subscribe to the creative cloud when that major upgrade occurs it will just be part of your package, you will continue to pay your monthly fee. You are probably aware that the creative cloud version has a few more features, and you also have access to Lightroom Mobile. Another big plus for the creative cloud plan is that you also have the latest Photoshop included in that package.
If you purchase Lightroom 6, you will have to purchase Lightroom 7 whenever it is available.
However, the price cannot be predetermined.
If you subscribe to Creative cloud, you will not have to pay anything extra for the latest version of any application apart from the monthly or yearly subscription charges.
Hope this helps.
Personally I use Creative Cloud - the full package. I would rate the Cloud Option as the better deal - I know there is a cost implication but it has worked for me seamlessly. There are many who will claim that this is an expensive way of doing things but the extra benefits far outweigh the disadvantages. The mobile apps are great especially as they are all joined up.