I made these Homemade Ninja Turtles Halloween Costumes the day before Halloween, but they were easy to make, fun, and was under $10.
First I made the shells: I went to a local grocery store (Safeway) and went dumpster diving for cardboard they were throwing out. You need to make sure that you have a large enough piece to cover the torso, and another piece for the back shell. Next, I went to buy some paints (yellow for the front, and a dark brown for the details on the back shell). I also had to buy some clear heavy-duty packaging tape and a large black Sharpe.
Then cut the torso and the back shell shapes out of the cardboard. Paint the front torso yellow, and add details with the marker.
Then I did the back shell, which was the fun part- because the cardboard is already brown, I didn't have to paint the whole thing, so I drew the shapes in pencil onto the cardboard. Then colored them in with the dark brown paint.
To attach the front torso and the back shell, I used two pieces of the clear tape for each shoulder (the bottom one facing up so it wouldn't stick to our clothes, and the top facing down) in order to make the "invisible straps" to hold the cardboard pieces on us.
The most time consuming part was the shells, but after you finish them, you can choose how much you want to invest into the rest of your costume. We just went to a thrift store to buy old green tshirts for the body (and wore them inside out so that the logos wouldn't show), and ripped up some old colored shirts to make the face masks and ties for our arms and knees. Originally we wanted to wear green tights, but didn't have enough time to go buy some, but perhaps you will.