Got mine brand new for $419. Figure the extra barrel, if OEM, is worth about $125-150. If other manufacturer, probably less than $100 extra. Kramer makes good leather. I would say no more than $500-550 depending on the "gotta have it" factor. Is this a pre-owned, NIB gun or is this straight from a dealer? I would think that you would need an extra recoil spring to use the 357 barrel. Recoil is somewhat different 357 compared to 40.