A few weeks ago I noticed that people had been donating to webpack in order to promote spam sites, so I thought I'd use the same tactic to promote something other than an online casino. I sent $10 to webpack on opencollective, which was then shown as a link back to the post on webpack's home page for ~14 days.
It looked like this:
The link was clicked 48 times, which gives me cost-per-click of about $0.20. I wonder how that compares with the price I'd pay for related keywords on Google Ads?
The CPC can be driven down further by making smaller donations ($3 will get you on the page at the time of writing), so it seems quite likely that spammers would be able to recover the costs of the donations fairly easily.