I would suggest getting a 450/400 3" NE #1 instead, the barrel contour is somewhat lighter than that of the 416 Rigby and it makes the gun much livelier, at least in my hands. And there's nothing like showing up at a range full of AR-15 shooters and then levering in 3.5" long shells
Mine thrived on 400gr Woodleigh softs over 4831, as well as 400gr cast bullets over Hogdon Trail Boss and AA 5744.
To answer your question directly: a 416 Rigby #1 would likely sell for $900-$1100 in my neighborhood, depending on condition. They're desirable, but not ridiculously so.
