Basically how much they have to lose.

Say for instance there are two vpn services. Both have a 100,000 users. One makes $1,000 a year off of advertising, and the other makes $1,000,000 a year($9/month). Now both are approached by a nefarious gentleman who offers them $20,000 a year to harvest their user's information. But every year there is a 25% chance people find out and your service is shut down.

Who takes the deal? Maybe the free guy, but very few people would risk a 1M/year revenue stream to make a little extra cash, but someone might risks a much smaller revenue stream for a comparatively bigger payoff.

That's not what was said. "Free VPNs are not to be trusted" does not imply "All paid VPNs can be trusted".

But to flip that around, what about adding payment into the mix has any bearing at all on the trustworthiness of a VPN provider?

Payment means there may be a viable business model other than sharing private information. Realistically I don't know how you can ever be sure, but I'd absolutely never trust a free VPN service.

It's not so much that they couldn't sell you out, but that if word got around that they had, it would be bad for business.