I recently wrote an article [1] about Artaic, a company that uses pick-and-place robots to build massive tile mosaics -- including backlit and QR code mosaics. They put individual tiles (1-inch or half-inch) onto an adhesive backing (12"x12") to ease installation. Swapping out the ceramic or glass tiles for currency would be a cool modification!

[1] http://www.hizook.com/blog/2012/08/02/artaic-revolutionizing...