They said “a domain that was making about $9k/year in ad revenue”, not “a domain that was making about $9k/year in profits”.
Also, even if it were making about $9k/year in profits, if that comes with large costs (be it labor or dollars), it still might not be worth it. Let’s say it costs $100k a year to keep that site making $9k in profits. That would be 9% return on investment. Good but not spectacular. Add in uncertainty about whether that site will keep doing that, and I can see such a domain not being worth much.
>Let’s say it costs $100k a year to keep that site making $9k in profits. That would be 9% return on investment. Good but not spectacular.
That's not investment, that's just the cost of upkeep. It's possible you simply cannot afford to keep up with that expense rate, but the fact remains that it's net profit. With a $100k investment and a yearly $9k profit, if you stop at the first year you lost $91k. With a yearly $100k cost and a yearly $9k profit, if you stop at the first year you earned $9k. No matter how you slice it it's a money-printing machine. The question is much it cost you to buy the machine, not how much it costs you to run it, because you'd be a fool to turn it off.