Redditor Pastlightspeed cosplayed Chel from the game Portal and put together an amazing Portal Gun out of cardboard and paint for less than $10. The Imgur set shows some details of the build, and the incredible fact that she ran a 5K while dressed in full Portal drag and carrying her sidearm.
The detailing is cut-out blue and black construction paper. Logos are printed on normal printer paper, cut out with an x-acto knife, and placed using adhesive spray.
To achieve the colored inner tube, I rolled a piece of blue construction paper and secured it on either end in the inside of a flat, doughnut shaped piece of cardboard. Rolled and secured on the outside of the doughnut is a clear sheet protector (a glow stick would've been perfect as the inner tube if the run had been at night)