多少 duo1shao3 acts as an adjective describing 钱 qian2. In Chinese, I think adjectives are kind of like verbs as well (i.e. 大 means big, but you don’t need to say something ‘is’ big – 是 shi4 would be unnecessary). When 是 is used with an adjective, it indicates a kind of emphasis (e.g. 他很高 = He is tall … 他是很高 = He IS tall (implying that you’re contradicting a statement someone made earlier)). I’m not sure what 这个是多少钱 means.