UnaBoard costs a small amount - $25 a month, plus occasional capital expenses (domain name, etc.). We have 1/10 the traffic of the SDMB, but are growing towards 1/8, so maybe you could scale it from there. However, I could have the same bandwidth as the SDMB for an extra $100 a month. And thanks to a large amount of unsolicited donations, we are getting a new server which (I think) is more powerful than the SDMB server. So at first blush, it seems cheap.
But then, you enter the coporate world, where things suddenly aren’t so cheap. Webservers at work which have less than half the power of UnaServer2 cost $10,000-$15,000 because they go for things like hot-swappable RAID, multi-CPUs (which really do nothing for a web server, but oh well), and even - and this is cool, I admit - hot-swappable power supplies. The only thing I have that is hot-swappable is bondage gear Fierra and I share (which was not paid for by donations, although there were many offers of free photography…) 
Then you figure that internal IT support is at $30 an hour with a 1.5 to 2.0 multiplier. So a woman-day ends up being an internal “cost” of $320 to $480. Spend 3 days trying to get a stupid SCSI driver to work right, and there’s more than $1000.
Pay some person to do 1 day of maintenance each month, and you end up with $4000+ a year in effective cost.
Now you may argue “but the person is on the Staff. Someone is paying for them anyhow. So there’s not really that cost, right?” Well, yes and no. Internal IT cost centers have to be tracked by businesses like any other expense, and what the Reader (presumably) sees each year in the pro forma is a line item for “SDMB support - $5000”.
So, while the Reader could do a very low-cost effort like I have in theory, in practice it’s hard to do. Some businesses pull it off, others are bound by intangible considerations, such as legal, security, confidentiality, or simply desire to have complete control of the server.
I mean, listen to all the people posting about how giving their CC number to the Reader will effectively result in it being skywritten over a Romanian uberhacker camp within 24 hours. Think your hosted server doesn’t get browsed on and looked at? I know people who own hosting services, and they snoop to their heart’s content.