I built this bird cage for about $10. It slides into the open cage door of the other cage.
Step 1: Materials
2 Wine boxes (the same size or one can be slightly longer. You will use these for the top and bottom) ~Free-$2
2 Cheese Log boxes (can acquire them from whole foods or anyplace selling higher end cheese, arriving early in the morning when they do production or calling the day before for holding. Goat cheese logs or small brie rounds come in "slatted boxes" you will use the slats for the door area) ~ Free
1 Roll of 1/2" hardware cloth (Do not get copper, zinc, or brass it is toxic for the birdies) ~$8
Branch (To block sharp wires at connection point)