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.
67 logos on the home page at the time of writing this, that's $6'700/m. While obviously not FAANG salary scale, it isn't to be sneezed at either.