Money per click is a function of keyword demand.
Advertisers bid on keywords on Google. Publishers/Authors get a percentage of those keywords.
For example, there was a time (I haven't checked recently) that keywords related to "refinance" were getting a lot of money per click because there was a lot of money in the mortgage industry for new mortgages. During the peak of that time, I was bringing in a total of almost $500 per month on variations on that theme with various online venues.
So, the short version ... writing a relevant article with high demand keywords will generally get you more money per click.