I think some of your numbers and assumptions are off a little bit. Although it does look like the signature market for lower level accounts (under senior member) is very depressed.

I would say that it would probably make the most sense for farmers to make 1-2 posts every activity period, then once they have enough potential activity to become a senior, they join a signature campaign for 'members' and post enough to become senior, then join a campaign for senior members (which appears to pay >0.001 BTC/post generally -- nearly $1/post). I have not really observed this, however I would guess this is what they are doing if they wanted to be more economically efficient with their time.

It is obvious that the farmers are making enough money to make it worth their while, otherwise they wouldn't be doing what they are doing.