OK, I've managed to find out for myself. So, for anyone who's interested, it comes from the 19th-century US slang of bundle meaning a wad of cash. This later became associated with betting, so you'd 'go a bundle' - big punt - on a particular horse. Now in the UK it has mutated its meaning to 'to be keen on', quite frequently used in the negative, so 'I don't go a bundle on xyz'.