It's worth exactly whatever someone is willing to pay for it, said Alice.
Can I suggest you do some research? Start with Google maybe.
OK. Seriously, it's hard to do satire or humour online, without risking upsetting someone and that's not my intention. This is a pretty rare book. It's one of the earliest caving books written by 'the father of cave exploration' and as a result is widely sought after by libraries, book collectors, ordinary cavers, etc. and especially those who have a combined love of books and an interest in caving. If you discover one for sale and fit that description you'll want to acquire it if you don't already have it in your collection. Cost tends to be a secondary issue - condition is important, but these days everybody is struggling and hoping not to be scammed online.
I've been a caver since 1962 and I was down a wild cave as recently as last August but now it's time to stop as my body isn't up to serious stuff any more. I'd like this to have a good home and to find a caver who loves books an appreciates it for what it is. If you wanted to read it, you could do that online if your archaic French is good. There are scanned versions out there. There are even translated versions of different chapters available out there. But if you know the feel you get when you hold an original then you'll know what I mean when I say it's a bit like finding a new bit of passage underground. It's YOURS.
It's hard for me to decide what I would accept as an offer because that decision is wrapped up in 'who is this buyer' as much as 'what will they pay' - for the right buyer I might be prepared to give a accept a different price compared to a commercial or institutional buyer. But let me say this - there seems to be a few of these selling online for about ?350-500 or ? or $ but I'm not certain about whether they are real or facsimile versions made in India.
I've had mine informally appraised as anything up to 1000 ?/?/$ but that would be under auction, and for a private sale I'd be willing to be more reasonable - especially if he profile of the buyer was 'right'. i.e - a caver who knows the value and a book lover/collector trying to build up his/her collection of such books.
And in any deal, payment would be by PayPal only as this protects both of us - and shipping would be extra at the actual cost of courier - international rates because I don't live in UK or EU - to guarantee secure and traceable delivery. (Fedex, UPS, DHL etc).
Hope this helps!