Outside of straight cash and ads, off the top of my head a user could give a website data, content, or computing power. Which, as I kept writing this, I've found aren't perfect alternatives.

Personal data collection seems compelling, since the data can be sold to hungry data brokers looking to optimise their ads, but tech-savvy users want to keep their data safe, either by using plugins to block ads and tracking, or by not using your website. And you'd also have to have no soul to do this.

User generated content gives users a reason to engage and return, and it also means you could save money that you'd have otherwise used to pay someone to make content. If you rely on this too much though, ethical concerns become apparent - last I checked, Reddit mods are unpaid.

Volunteer computing could maybe lower costs by offloading some server calculations onto volunteer's computers when idle, but I don't know if it could even be used for that. It's probably a non-starter for websites, too; to a user it would seem like your site was asking them to install a crypto miner.

... this comment is getting too long and doesn't really have a point. But I can't let the 45 minutes I spent writing it go to waste so easily. Hm... what if I combined all 3 ideas?

Yes, a website that asks you to volunteer idle computer time to train an algorithm that can both be outsourced to other companies and used to analyse your personal data, which itself can be given to other companies and used to reccomend you posts you are more likely to comment on, adding value to the website! Surely this has none of the flaws that I described before.