This is an incorrect assumption. The meaning depends on the context of the sentence (and the main verb if there is one). In this sentence, you could swap the order and it would still mean "how much". (Just like in English, you can say "how much is your skirt?" or "your skirt is how much?")
Here, the subject is singular. Barring any main verb context, it would be awkward to mean "how many is your skirt?"