They do cost an eye-watering amount to print - if people were prepared to pay £15 for one, there would not be an issue, but low run posters like these are expensive.
It _did_ occur to me that something like this is an ideal Kickstarter project, a very low one with something like a £500 goal for the first map, and then perhaps we can go on to do others as stretch goals, or seperate Kickstarters. That would take some thought though...