Probably the first question is what needs scuplted? New arms only or complete sculpt.
Assume everything needs to be sculpted, and scale down from there if need be. IWM usually charges $800.
Also, assume Psycho would be doing this sculpt because it would be amazing if Psycho took the turd that was the current Phoenix crusader and made it into a gem, like he always does.
For those interested, please see his other amazing mod work with the Rifleman X3, Mad Cat Mk II 4/5, the new Marauder Variants, the battle damaged Mortis, the Warhammer 8K and a whole slew of others...