There are/were places where you could get really cheap (or free!) backlinks, but most search engines got wise to that trick, and those backlinks (links from spammy sites with little organic traffic) aren't very valuable anymore. To get a link on a reputable standalone website, $100/m seems cheap to me. What better alternatives are there at this quality level? A B2B partnership/sponsorship? You can easily spend 5 digits for that.

> $100/m is cheap for a back link? (honest question - I've no idea).

Depending on the backlink and ambitions for the website in question, I personally don't see that as unreasonable.

They have a lot of money to launder. Better to spend some of it to reduce exposure, and increase SEO ranking.