Dan25 has the right idea if you are going for effectiveness and are not in a noise restricted area. You can find a 20 or 12 gauge single barrel shotgun for around a hundred dollars. Cheap ammo will to the trick under 50 meters and it is about 25 cents a round...but you can get crows down almost every shot. Pellet guns (especially high performance ones) are precision machines designed for sniper like tactics. A solid shotgun is more like a heavy artillery piece for birds and can be effective without having to use rocket science (wind doping, drop, placement, humidity, and a 150 dollar scope). I LOVE taking small game with my Marauder .25 but it is still a chance of circumstances and technique. If I have an hour to get out in the woods and fill my bag with 5 squirrel for the dinner plate I'm using my 20 gauge. This might go against the grain on such a site, it is the truth as I see it. You can get a decent spring gun for 200 dollars but with open sites you are going to need luck and skill and you are still probably going to miss a few. With a 100 dollar shotgun you need 20 dollars of shells and keep the beed on them and you will be having crow pie in short order.