But not "way more than you would think" where "you" includes readers such as /u/agmemnonymous and myself.

IMO you should be able to guess >5% just from the OP image. The OP implies that corporations actually used sawdust as a substitute, which implies it was a profitable substitution, which implies it was worth it to set up an entire supply chain for bagging sawdust in sawmills, outbidding other parties interested in industrial quantities of sawdust (such as paper mills), shipping it to cereal factories, mixing sawdust into the mix, and trying out ways to make it homogeneous, not to mention the risk of customers noticing and switching to alternatives with less sawdust.

That said, nobody uses sawdust anymore because it's too expensive. Hay, straw, and chaff are much more common sources of cellulose, also known as dietary fiber. Most people in the western world would be healthier if they ate more sawdust (assuming it has been produced in a way that didn't introduce toxic pollutants).