A ninja costume is incredibly easy to make on a small budget. The most complicated part is the mask, which you can make in only a few seconds with the aid of a regular t-shirt.
The Mask - Who's that ninja? Well, you aren't supposed to know. Every ninja wears a mask, and you can make yours just by using this folding trick on a t-shirt: Make a Ninja Mask from a T-Shirt
The Clothes- For an authentic-looking ninja costume, a long sleeved turtleneck is the way to go. Tuck your mask into the neck of the shirt. Use any pair of black pants for a traditional ninja.
Storm Shadow - For a typical Storm Shadow costume from G.I. Joe, you'll need a white tank top, white cargo pants (bleach a pair of khaki's), and a pair of white boots. White boots can be tough to find, so employ a can of spray paint to get the look. For the wristbands, you can learn how to make wristbands out of socks.