Under $100: the marketplace tier.
Fiverr gigs, bulk marketplaces, and "200 links for $50" packages live here: comment spam, profile links, expired-domain networks, and PBN posts on sites with no readers. SpamBrain likely devalues most of it silently, so the usual outcome is nothing at all: money spent, graphs flat. At volume, the pattern itself invites a manual action.
The tell is inventory. Anyone who can quote a price list of hundreds of available placements is probably selling from a network, and networks are what detection systems are built to find.