I paid 32$ for my piece of coroplast, 48in by 96in. It was a little outrageous.
However, double check your home depot. Ask for plexiglass instead of coroplast. They should take you to an area that has both. Most people at home depot, I've discovered, do not know what coroplast is. You might have to buy more than one sheet depending on your project.
As an alternative I used vinyl (laminate) flooring for Penny's cage. I now use plastic cube sides that I found at dollar general. You might be able to find them.