I made my own cat scratching post out of sisal cord and a poster tube. It cost more than just buying one.
50' of 3/8" sisal rope ($0.25/foot) winds almost perfectly around a 30" by 2" diameter poster tube ($3.33.) My daughter and I had a wonderful time wrapping the tube up, securing the ends with binder clips, and then using a strip of old bedsheet to hang the scratcher from a doorknob! Rubbing a little catnip on it encourages our Maine Coon to scratch away!
Total cost invested was around $15.00.
Even if reusing the tube, binder clips, and sheet I can easily find an equivalent scratcher for less. Even using cheaper cord only changes things by a few cents.
I'll put the two side-by-side and we can see the wear over time and judge which is more popular. Perhaps my cat will appreciate the love and effort that went into making him a toy.
Petlinks Hanging Rope Relief Cat Scratcher via Amazon