Font has different shape, space between letters, bold or not, and so it can makes a difference. Actually one font is quite popular but isn't free anymore, it was ecofont

looking on internet, I found some articles ( including wikipedia article ) , and Century Gothic seems to be a modern looking and could save 30% of ink.

but to be able to implement a on-the-fly font replacement of the pdf file with greencloud printer, I had to look at more tools and review

someone reported this link but it is dead now !

the quick conclusion is this

"Our first choice of ink saving font for body type is Century Schoolbook, which the University of Wisconsin just picked for its publications because it uses less ink, and which the U.S. Supreme Court requires for readability. It is an excellent font and it is no wonder that it is the standard for such prestigious organizations."
I wonder if it would make sense to implement in greencloud a little tool to compare the % of ink covering the page, so it could be used to review different font based on the pixel values all over the page ?