$75 for the gun, up to $125 with scope, sling, and accessories in excellent condition. Midway USA had these on close out a few years ago for $70. Still, if it's a decent inline in decent shape, it's still worth $75-100, even used.
Best left to a gunsmith
100-400 USD
Cabela's or Dixie Gun Works would be your best bet, or gunborker website.
The exact value of a Remington Sportsman 12 Magnum actually depends on a couple things. Some of these things would be the condition and age of the Magnum.
300 or so
$50-$100, depending on condition.
The National Wild Turkey Federation 300 Weatherby Magnum is a type of hunting gun. In mint condition, it is worth thousands of dollars.
You need the services of a gunsmith
ABOUT $1500. depending on exact model, condition, scope, etc,
50-150 usd
Depending on condition, $400-$475.
About $600-$700, depending on condition.