There is one self-hosted plan:
https://invisioncommunity.com/buy/self-hosted/
With that option, you can choose which modules you want with your license. You get access to the software, but need to run everything yourself. You get to run the software forever, but updates and support are included for 6 months. After the 6 months, you need to renew your support/updates for an additional 6 month period.
The various cloud plans are at:
https://invisioncommunity.com/buy
With the cloud plans, it's 100% hosted by IPB, so there is no software... you access it like you would Gmail in that you simply use the solution. IPS takes care of all of the underlying "stuff" such as managing hardware, backups, firewalls, etc. There is a matrix on that page that shows which features are included with each plan. The pricing is shown with the monthly or annual cost. (Paying by the year is cheaper.)
There is a smaller Beginner plan that is not included in the matrix as well....
So if you need just the forum app without the other things such as commerce, blogs, files, pages, etc... that could potentially be a starting point for you as well.