"And if you thought you saw anything there, understand that there are more illusions in a tinker's cart that there are in the rest of the world. And that's saying a good deal, for it' a shadow of a dream."
This is from ”The Story of Mona Sheehy" by Lord dunsany.
I can't understand the meaning of
"that's saying a good deal"
I am glad if somebody strong text would kindly teach me.