All of the materials you need should be available from your local Home Depot or Lowe's. Remember, it may be a good idea to buy a little extra, in case you make a mistake. Many of these stores will allow you to returned unused materials. I put an estimate of what I paid for the materials to give you an idea of what to expect.
* Many home improvement stores will cut your wood for you. This is a really good idea as far as the plywood goes, especially if you need to fit your materials into a car. Either you or the store will need to cut the plywood into a 4'x2' piece, 4'x10" piece, then 2 2'x1' pieces (cut a 4'x1' piece, then cut that piece in half). That will leave you with a 4'x4'2" piece, which you may want to keep for your next project.
** The castors are optional, but recommended. If you do not plan on ever moving your cage, you can save a few bucks and get rubber feet instead of the wheels.
*** Wood trim can make a good lip for shelves, so it's harder for the cats to knock bowls off of them, and also for covering the edges of the chicken wire on the outside of the cage.
**** These instructions do not cover painting, so if you would like to paint your cage, you are on your own.
***** If you are planning to keep an open litter box in the cage, it is a good idea to install screen around the bottom of the cage to keep the litter from splashing all over your floor.
So you are looking at around $100 to $125 depending on what you but and sales tax.