Green’s Dictionary of Slang

how much? excl.

an excl. of incredulity, a demand for further detail or information, esp. when what has been offered seems unbelievable; it need have no ref. to price, but is delivered in response to what the listener considers to be a far-fetched statement.

[UK]F.E. Smedley Lewis Arundel Ch. xxxiv: ‘Then my answer must mainly depend on the exact height of the principles.’ ‘On the how much?’ inquired Frere, considerably mystified.
[UK]Hotten Sl. Dict.
Wodehouse ‘An International Affair’ in Politeness of Princes [ebook] ‘Sal how much?’ ‘Ammoniac. You'd be awfully bad’.
[UK](con. WWI) ‘Taffrail’ ‘A Little Drop o’ Leaf’ in Little Ship 245: ‘Here’s your meat card, Mister Fatty. Goodnight!’ ‘Mister ’ow much?’ Billings demanded.