I think that "Try before you buy" is the most important aspect of what you get from a guitar dealer versus from a living luthier. A guitar in the hand is worth two... But that's only one type of sale. In this case you can have a sense of what that luthier's prices are, the expected wait for a bespoke instrument, etc. In other words, relatively easy to determine the cost/benefit of the transaction. HOWEVER, if you are looking at older/historic or famous builders, it may not be possible to accurately assess what they are worth. Sure, there are other ones out there by the famous builder, but condition, demand and just "wonderfulness" of the given instrument can affect the value. So then it comes down to just what are you willing to spend on a given guitar on a given day. And collectors and players may have different criteria for making a purchase.
I'd say shop carefully, try lots of guitars in different settings, and get to know a dealer or two. A bargain is a bargain, but love is love!
"…that all-pervading talisman of Spanish pleasure, the guitar." -Washington Irving