Yeap, it is Free!

“The cost of postage is offset with the help of relevant, targeted, unobtrusive advertisements, which will appear on the back of your Gmail Paper prints in red, bold, 36 pt Helvetica. No pop-ups, no flashy animations—these are physically impossible in the paper medium.”

Well seems to be another Google April 1st prank along with Google TiSP. So the the 20% of time that Google employees get to think creatively is spent mostly on April 1st gags? They all seem so well thought of and made!

Google Paper