It's probably around £100/year for a basic forum with hosting and domain etc
Bushcraft UK server fees are around £2.5k, then there's domains (multiple even though we generally use the one)plugins, tech help, Amazon SES and software. Some years it's as little as say £4k and others it's been £7k in pure running fees.
There's other software out there that's far more expensive and leans to large commercialisation of the forums.
When we first started Bushcraft UK it was running phpbb and it was cheap as chips, but I soon realised that it was very limited for a growing forum. If it's a small group and you want an easy life then that sort of thing is fine, and the learning curve isn't that big. The learning curve on paid forum software and the server side of things is much steeper the bigger you get.
I'm more than happy to answer any questions you might have, give my opinions etc, drop me a message...