50-500 USD depending on specifics
The Hopkins and Allen company was founded in 1868 then merged with the Forehand Arms Co in 1901. Out of business by 1917.
The Hopkins and Allen company was founded in 1868 by S. S. Hopkins, C. W. Hopkins and C. H. Allen. The company experienced financial difficulties and in 1898 was reorganized as the Hopkins & Allen Arms Co. A fire in 1900 hurt them and in 1901, they were forced to merged with the Forehand Arms Company. The manufacture of Hopkins and Allen firearms ceased in 1917 when the plant was taken over by Marlin-Rockwell Corporation for the war time production of Browning automatic rifle components.
It should be marked on the barrel or frame.
Caliber can not be idenitifed by sn. Take it to a gunsmith if it isn't marked on the frame or barrel.
10-100 USD or more depending on specifics.
Blue Book of Gun Values
Impossible to answer without a detailed description of all markings, features, finish and caliber.
.45 calliber
Depending on condition, $25-$100.
Try the blue book of gun values
41 or 42 ? about 350 $
Try Blue Book of Gun Values