From what I can tell, The upper bound on price for any site making less than 100k a month is 24 months of revenue, but the more common is around 12 months.

The buyer takes on substantial risk because it's easy to fake the numbers, and google updates can tank the site at any time.

Also, most sites will require maintenance/upkeep to keep earning, or they can tank quick. Even if they have got evergreen content, without updates google might drop their search ranking.