That general rule of thumb is bullshit!
I have a very small network of sites up around one niche, less than 20,000 per month for the primary KW, each and every SMALL content site I have around that niche banks about $5 a day which isnt bad when your talking run of the mill 1 to 2 days from start to finish WH sites. You have to remember that your going to rank for other keywords besides your main KW, for example "my Cock" might get 40,000 searches per month where as "my cock in your mouth" might get 10,000 per month and "my cock in your girlfriends ass" might get 9,000 and so on. You will never build a site and only get traffic for a narrow keyword, in fact as hard as you try usually its the other way around.