'How much' here has nothing to do with number: it is a fixed expression referring to price.
Furthermore, in the examples you cite, it is not a subject, but a complement. The subject is the noun phrase following the verb, which naturally is the part of the sentence that determines verbal number.
In the case of 'a hamburger and chips', on account of a linguistic phenomenon known technically as synesis, combinations of foodstuffs, when regarded as parts of a single dish rather than as separate items, are generally treated as singular (i.e. the question refers to the price of the entire dish, not of its individual components.)
I trust this makes the matter clearer for you!