Virtually none since it has been fired. Value range of about 350-500 USD.
The collector rifle can be found in the collectors edition or in the Recons Operation Pack
100-1000 depending on specifics
The N7 weapons are only available through the collector's edition or preorder. Collector's Edition: N7 Hurricane (SMG), N7 Crusader (Shotgun), N7 Valiant (Sniper Rifle), N7 Eagle (Pistol) Preorder Bonus: N7 Valkyrie (Assault Rifle), N7 Defender Armour (Body Armour)
The assault rifle you get from it is pretty strong other than that there isn't much special about it. Tip this into google "See the Mass Effect 3 Collector's Edition Unboxed, Complete With Robotic Dog! GameFront" and you will see that it all you can get!
100-1000 usd
Halo 2 has the gravity rifle in it but i (think) you have to have the custom edition to get the gravity rifle.....
100-700 usd
Around $900, dpendiing on the exact version and condition. Very nice rifle.
In general, yes. IF the rifle is a collector's item (not all old guns are) it can lower value greatly.
Your rifle if it is unfired(new in the box) with the original box will bring 675 dollars.
100-300
You will have to contact the Weatherby Collector's Association.