The plastic that these organizers come from is what sign-makers use. However, they are SO hard to cut without the proper tools. I have tried an exacto-knife, scissors, and even a paper cutter (and the blade dulled after just a few!) Sign-makers have special tools that go down between the flutes of the coroplast that makes the cuts smoothly and quickly. However, most sign-shops charge quite a bit for cutting the boards and a more than doubled charge for the boards than what they paid.
Using cardboard is fine... if you have LOTS and LOTS of it and the time and hand-strength to cut it. The same goes for using mat boards. The strength of the mat board would concern me, too.
For me, I have an large stash of fabric, so I have about four hundred coroplast boards I am using right now and will be needing about a hundred more to finish up. Then, I am going to move onto my fat quarters and start them.
I also have been watching how they secure the fabric at the LQS and have used their folding technique to just fold my fabric around the plastic. It has worked well and has saved me having to purchase anything additional to secure my fabric.
As for the cost of these... I like being organized... I like having my guest room look nice instead of haphazardly stacking fabrics. I also love to just go lay on the bed in the room, look at all my fabrics, and dream of the next project I am going to do... someday. It is worth it to me.