You can check out similar sales on flippa.com - ad revenue does not last forever, even if it’s existed for years. And revenue is very much not profit, you could create a site and get $100/day in ad revenue tomorrow but it would cost you $200 in ad spend.
What does the spend go to, besides hosting costs?
Advertising costs, to drive traffic to your site(s).
So, spam?
Surely generating spam has a cost.
You can’t just send tens of thousands of emails via your typical email service.