you can get a brand new 454 7.5 Super Redhawk, for $799-$825.
check gunbroker out, for comparable prices.
they make a book for a blue book value on guns, but it would be hard to put a price on a custom.
I'd try and get the gun at the price of a new super redhawk, or lower.
i've been trying to get a .500 Smith and Wesson, and the one I found had a leupold scope, and came with a different set of grips and a shoulder holster, and finally got the price down $300 less than what the guy wanted, but I really only wanted the gun, and for what he wanted, I could have paid $100 more, and got a brand new one.
this link is a discussion on converting a redhawk .44 magnum to .454. one poster states that Ruger doesn't recommend doing the conversion. That it would be unsafe. that the round is too big and could destroy the frame of the gun.