1928, it will be date code stamped on the barrel. See the Manufactured Date page on the Remington Society of America for details.
Depends on the model.
Which model 12-a, 12-b, 12-c, 12-cs? But it's mostly about original condition, condition, and condition. Without that it's 100 - 1,500.
1914
1913
1920
1919
1920
1915
1912
1920
1912
1920