You can ask
How much do you agree with the following statements?
and the answer will be "very much" or "a little" or "not at all", but it won't be "7" or "4" or "0".
On the other hand, if you ask
How much money do you have?
the answer can still be "very much" or "a little", but it can also be a precise amount ("4 dollars", or "33 euros").
You can substitute "To what extent" for "How much" only in the former case, when you are in fact asking for the extent of something (the extent of your agreement to the statements). In the latter case, you aren't asking for the extent of something, but rather for the precise amount, so "To what extent" doesn't make sense in that context."
Note that you can always replace "to what extent" with "how much", but the reverse is not always true. "How much" has a wider range of uses, and some of those don't equate with "to what extent."