Hacker News sells inline advertising. Job posting adverts crop up pretty much daily on here which are adverts. It's a recruiting tool.

As for expenses, HN seems like a lean site and likely doesn't require a ton of resources. I think there is only 1 full time moderator, dang. I have no idea whether YC makes money on HN or not, but I'd be surprised if it didn't at least break even.

Also the free advertising for the brand of Y-combinator. With the reach HN has these days having your name out there right in the domain means that the on balance positive reputation of HN rubs off on your enterprise. It's basically a huge 'sponsored by' byline. This doesn't necessarily translate into a net positive for a company, but in this case I am willing to bet it does.

It's honestly a fairly transparent transaction, and much preferable to the opaque world of on-line targetted advertising. I like how my ad-blocker just sits here twiddling its thumbs.

Forums don't require much resources from a technical point of view - most of it would be for moderation, but even then, there are usually enough passionate community members out there to do it voluntarily for free.

You can make enough money to sustain the thing with a one-time joining fee or unobtrusive, directly-negotiated advertising. You just can't make unsustainable VC-level money.

Reddit also sells super-upvotes (called "awards") that you can buy individually or as a component of a premium (ad-free) subscription. Not sure how much revenue that produces compared to ads, but it's definitely a component.