Enough to pay for it … if it’s good enough to be worth paying for.
I’ll give you an example. I use a site called lacemarket which is a buying/selling site for a niche hobby of Japanese street fashion. It will never be popular enough that enough people would be using the site in order for them to make enough to pay for hosting the site.
So they’re forced to run ad’s cause they have no other way to keep the site up. The owners are also not taking a percentage of people’s sales so they can continue to bring in people who want to use the site. But in order to not take money from the sellers, the only other option they have to keep the site running is ad funding.
It sucks but its too niche to do it any other way.