Green’s Dictionary of Slang

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.

1852		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.
1864, 1867, 1870		Hotten Sl. Dict.
1928		(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.