What evidence? This isn’t a rhetorical dismissal, but genuine curiosity.
The prices paid on marketplaces for sketchy lead generation, backlink, SEM, MLM, PPC and scam delivery services (human and semi-automated spam). The prices spammers pay for ancillary services they need, like hosting and Captcha breaking. And cost per click rates for Google ads give some indication I think.
When insurance companies are paying Google $40+ per click, it’s not hard to imagine they’d spend $0.01, $0.10 or even $1.00 to deliver an unsolicited email to a potential customer.
Also, in situations where there is already a delivery cost (such as a form that requires a Captcha or phone number), spammers often use MLM-style scams to dupe other people into paying that cost for them. That’s what a lot of those “my sister makes $x an hour” scams are about.