I have the same ladder and over the years have replaced all of these inserts. Early on they were about a third of what we are seeing now. I don't think the original ladder is available anymore so replacement parts have gotten expensive. Also, I haven't found a replacement that is easy to install the way the original is attached. I still have a couple of spares so hopefully the ladder will outlast my ownership.
And the suggestion to fill in the crack with something is not a good idea as a failure of the insert will cause a bad fall and the crack is the first sign of that failure.