It depends what the book is. If a mainstream book has been selling for around $2,500 in a certain grade fairly steadily, with gradual increases in price as opposed to spikes from movie tie-ins or bidding wars, then you may not lose too much even if there's a downturn—maybe 30% (not counting selling fees).
But if you're talking about something that not too many people collect, or something that cost $2,500 only because it was hot or because a couple of guys got into a bidding war, then you could lose a lot more than that. I have seen even high-grade GA superhero books lose more than 50% of their value (when that "value" was the price paid in a bidding war).
Unless, a book is destroyed or stolen, though, then you're not going to lose everything. I generally figure I'm risking about 50% of what I pay for any given book. If I buy a $10,000 book, then it might drop to $5,000, but it isn't going to drop to $0.