Have seen them range from $550 to $800 for nice used JM's. If you want to mount a scope at some point, make sure it is drilled and tapped, as the old "waffle top" ones are not. Not an issue of you only plan on using the open sights, and is a neat feature.
Brand new, they run around $720. Hearing that some of the Remlin issues have been resolved...
I bought a really nice, older JM from a collector selling off his duplicates for $750 two years ago, as I could not find another one in nice shape at the time. Bought it to be a saddle/camp rifle for the wife, who doesn't care for my 356 or 358's recoil levels.