Really depends on traffic.
Most of it comes down to hosting costs.
More traffic the more costs.
Initially for good performance you get a decent server, either a VM instance or a dedicated host with a decent pipe, most guys starting out go the per transfer instead of the per MB commit, that way they can get a larger pipe (usually shared instead of dedicated)
More traffic that comes in the more bandwidth is consumed, per user it really isn't much and the biggest hit is going to the the IO on the database(s).
most of these sites aren't built with scaling in mind. They're built out of love for the hobby (in this case our C7's) so they don't think of what it takes to operate a site that gets 100,000+ hits a day (not sure what this site gets but would think it isn't a ton comparatively speaking).
Using out of the box forum software can be an issue as well, as it produces unnecessary IO & bandwidth consumption as well so custom development is generally needed to scale. Which most hobbyist aren't equipped for.
If you went out and got a VM and put a site up it would cost you about 4 dollars a month up until you started getting any real traffic. Before you know it you could be on the hook for hundreds in storage/bandwidth costs.
(instances on AWS and other VPS's use a storage/RAM per GB model, more traffic you get the more of both you'll need)
This is very general and I'm not being overly specific and do not speak for this site. they may self host with dedicated gear in a colocated datacenter with dedicated bandwidth for all I know and in that scenario you would be looking at something similar to this, a full cabinet is about 499+ then electrical 20A 120V x 2 400+ and bandwidth lets say 100Mb synchronous dedicated pipe is 200 - 700 depending on carrier, peers, quality etc.
so just to answer the question.
anywhere from 5 dollars to 2000+ a month.
some of these hobbyist are too proud too, another forum I frequent the guy that ran it was asking for donations every single month, I offered to host the entire site for him for free just because I liked the site. He refused and would rather beg for donations to keep the site running.
Others sell, I can't blame them, if you had a site that you really didn't have the technical prowess to scale with your user base, selling it to an org that does is idea.