Why not just provide a reasonable estimate, with a "price assuming this goes as expected", what can go wrong and a price for fixing the various common issues encountered along the way, and a worst case scenario price?
However, in my experience, if you tell a client "This will cost you anywhere between 50k and 60k depending on what goes wrong" they will immediately assume they're paying you 60K since it's pretty easy to justify that extra 10k given your language. Regardless of whether your intentions are gold. If it happens to work for you though, then go with it ;)
This is why it's hard to get a real budget out of a new client. They assume (and often rightly so) that you're knowledge of their budget dictates your quote.
Fortunately, I have found once clients are used to regular progress, they're more accepting of variation, and willing to accept estimates based on real statistics.