or How much more expensive is your phone than mine? Are those questions well-formed? Do i have to use much here?
I cannot give chapter and verse on this answer. I learned my grammar from Latin. Latin has something called *ablative of (**degree of) ** comparison*.
Manus meae multo breviores sunt quand tuae. My hands are much shorter than yours.
Multo is the ablative of the adjective multus. So literally, it means ...shorter by much. English has similar devices.
My phone is more expensive than yours by far
My phone is cooler than yours by a mile
These 2 are adverbial phrases and postmodifiers. Much is a premodifier. By a mile and by far can become premodifiers with a little modification.
My phone is miles (or far) more expensive.
You questions ask how much shorter or more expensive. So this usage can be called ‘degree of comparison.
Health warning⚠️: The Latin is correct, but I have so far not found evidence for the English.