A working Finnbear will go for around £400-450, so I'd say £150-200 for just the action, with magnum actions being marginally less popular. These are Sako at their engineering best and a great basis for a nice semi-custom job.
I recently paid £140 for a good 695 action and mag' which I considered a bit of a bargain - floorplate actions tend to be less sought after for some reason.