Between about 1870 and 1914, H&A was a major sporting arms maker, producing reasonably quality revolvers, small caliber rifles, and shotguns. There is a mild collector interest, but the guns are not very expensive, usually running under $200 even in top condition. The exception is the so-called "parrot beak" derringer, which can run over $2k in top condition.
In 1914, H&A got a contract from the Belgian government to make their Model 1889 Mauser rifle. 8000 were made, but the Belgians were unable to pay and H&A went bankrupt. Recently, divers discovered the wreck of a ship off the Belgian coast and brought up rifles marked Hopkins & Allen. Apparently, the Belgians never received the rifles and so did not pay. The loss of any great number of those rifles would also account for their scarcity on the collector's market today.
Jim