What do you consider expensive?
My coroplast is probably the highest price that I've seen on here and it was a 4'X8' sheet for $35, but I didn't want to drive to a less than desirable part of town, and I wanted yellow, so I was willing to pay the money. A sheet that large though can make a tray big enough for a 2X6 cage with 6" sides. Combine that with the 28 grids that I'm going to use (my lid is made of cube grids also) and a pack of cable ties from the hardware store, and I still have a cage 2 to 3 times larger than any pet store cage for around $60-70. Many forum members found the same size sheet of coroplast for $10-20, so their cages if made the same way would be around $50.