I wouldn't say easily over 50%. For example https://www.varonis.com/blog/inside-world-insider-threats-pa... says roughly about half, and that sample might be biased as i assume its easier to take someone to court if they did it for money.

Still your point stands that money is a common factor. However you still have the problem that bribes usually aren't a one time payment but an ongoing thing, so a purely financially motivated adversary would take the total value into account.