I can see a Mondonico fetching $4k quite easily. They're rare and getting rarer (in the States), they're handmade in Italy by the family they are named after. Plus, if it's vintage, the age will add to the base price and just keep adding as the years go on.
A complete Mondoncino with Campy (my guess would be the LBS isn't sparing on quality) - yeah $4k is pretty average.
You also have to consider that the builder is figuring in time, effort spent, and cost of handpicking the components.
I think $4k is within the reasonable range of a custom built Italian artisanal steel bike on this side of the Atlantic.
That's about what I spent to build mine.