It depends on how much there is to blue and how bad it is to start with...
For example - a Remington 700 action and barrel compared to an old Sako Deluxe where every piece is steel (trigger guard, floor plate, etc.) and is highly polished and blued ...
... now imagine the 700 barrel and action are simply worn a bit with no rust pits and imagine the Sako which had a high polish blue is all rusty and pitted.
Probably $150 to do the 700 and $400 or more to do the Sako...
it is all based on the number of hours of preparation, disassembly, cleaning, polishing, polishing, polishing ( did I mention polishing), cleaning, bluing, oiling, cleaning, and final assembly ... the actual bluing process only takes 30 minutes.