In good shape? Hunted to extinction, much like its stainless twin the 3" Model 65.
Are we talking about the one in the picture, or is that just illustrative?
A 3" Model 13 Round Butt, the FBI model, in NRA very good to excellent condition, with the original box numbered to the gun, paperwork and tools, is a collector's and a shooter's piece and will fetch a premium. Starts at perhaps $650 and just goes up from there.
Unfired, new-in-the-box? Wow. Collector only and probably starts at $800, though if you told me you saw one sell for $1000 I wouldn' be surprised -- these were the last FBI-issued revolvers before the world went semiauto, and have the added mystique of that legacy; also, they're just great fighting guns.
Shooter grade, NRA good to very good? Anywhere from $400 to $550, all depending on the specific piece, location, seller and buyer.
And there's always the guy who walked into a pawnshop in the middle of nowhere and found a mint one for a song to throw off the averages (I am still waiting to be that guy).
Tell us more about the one in question...