XenForo only costs monthly for a hosted license. Which, even if that is what the site currently uses, will not be what it uses post acquisition. A one off license may be purchased very reasonably.
Hosting can be far cheaper than 300 a year, or far more expensive. The one upside to less traffic is that less powerful servers are needed.
I would probably consider switching us over to an open source forum software in future, but for now as is, it wouldn’t be super expensive to keep things running as is.