Hi Jonathan,
I can't help you with a car I'm afraid but with regards to answering your question the cars are quite rare now so I would advise you are flexible in your requirements. I wouldn't for example get hung up on a particular colour, if you find a good car then buy it. The sad fact is there are not that many excellent original cars out there, and the few that are out there you'll find the owners don't want to let them go. Therefore the real answer to your question is, if you find one and you desperately want it it's worth whatever you are prepared to pay for it! Looking at recent prices something with an MOT, useable daily but needing some degree of cosmetic work seem to be in the £1k to £2k region (though the state of the chassis on these cars is another matter - really strongly recommend giving the underside of any purchase a good inspection. Take a torch for looking in nooks and crannies or better still get it up on an inspection lift!).
Cars I'd describe as very good - but not necessarily perfect then sit in the £2k to £3.5k region, and I haven't seen anything recently sell like it but something truly immaculate could probably break £4k. Occasionally dealers in magazines like Classic and Sportscar get hold of one and seem to ask in the region of £5k - £6k. I've never been to see one of these in the metal but I tend to think they are really a £3k - £4k car with dealer premium added.
The others on this forum may well refine my view with better opinion / evidence. If you were prepared to have lhd, and scour France you'll widen your potential net massively and might get something cheaper than above. There's French residents on the forum who can guide you through the paperwork trail, and may have access to decent cars too... Best of Luck!