I can give you a ball park firgure.
The ten round mag is worth $100 to $150, $75 to $100 for the three shot mag. The 05 factory scope mount has become very rare. New from HK is $400 and sometimes that is with out the rings.
As for the 940 depending on condition I would say that it is worth $1000 to $1800.
My Uncle has two HK940s he bought one for $995 and the other for $899, this was three years ago, and in small gun shops. That is where I get the $1000 figure.
If you search gunsamerica.com , or gunbroker.com 940s are listed for $1500 to 2000. However I think when they are over two grand that is asking too much.
I would say insure the gun for $2,225.
$100 for the 3 shot mag
$125 for the 10 round mag
$400 for the 05 Mount
$1600 for the gun.
940 is a great rifle, and they are getting hard to find.
Chris