From CosplayWiki

One of the cheapest and most easily avaliable material to make cosplay costumes and props out of is cardboard. Inexpensive (Usually free) and easy to work with (all you need is hotglue and scisors) cardboard is the ideal prop for cosplayers just starting out cosplaying.

CosplayWiki Tutorials

Featherweights' Cardboard Keyblade
Featherweights' Cardboard Katana

External Links

http://forums.cosplay.com/showthread.php?t=97106Featherweights' Cardboard Helmet
http://forums.cosplay.com/showthread.php?t=89839Featherweights' Ichigo Hollow Mask
http://forums.cosplay.com/showthread.php?t=71070Featherweights' Kunai/Shurkien