When talking about containers that hold powder, I tend to focus on the early carved powder horns because they are so attractive. But there are later flasks that are interesting and have value, as well as those that are a lot more affordable in some cases than their carved ancestors.
As the horn began to fall out of favor, other flasks made from leather, copper, brass, and tinned iron began to be used. Some of the brass and copper flasks can be die stamped with hunting scenes, foliate designs, as well as some military flasks with a bugle, “US,” and other patriotic motifs. Some arms makers had their own flasks made, one being the Massachusetts Arms Company. They made a flask to go with their pistols with foliate designs, crossed flags, an arm and hand with a pistol, marked "MASSACHUSETTS/ARMS/COMPANY/CHICOPEE FALLS." on both sides. In good condition these small flasks are collected widely. Pistol flasks that are plain or with a hunting scene can be purchased fairly cheaply today but are a good way to get into collecting this type of material. Other pistol flasks were made for other companies like Colt. Some of those can bring high dollars depending on what gun they are for. For example, a flask made for a Colt Paterson revolver can cost almost $5,000.00 compared to a small hunting flask that may cost $40.00. Like most collecting fields it pays to do a little bit of research. Colt also had pistol flasks for their dragoon series of revolvers, as well as the Model 1849 pocket pistols and later martial revolvers. Those can be in the few hundred-dollar range to $400.00-$500.00.
There are also rifle flasks and the examples that have the most value are military, not civilian made for hunting. There are a variety of examples from the bugle eagle flasks to the so-called “Peace Flask.” one of the dullest looking examples is for the Hall rifle. It is made from tinned iron that is soldered together and then japanned in black paint. It was a dual-purpose flask in that it held not only powder, but it had asecond compartment for lead ball with a spring, lever, and spout for each side of the flask. It was slung with a piece of leather sewn around the middle with a leather strap that buckled to a leather tab sewn on the front. These are very rare and can sell in the $4,000.00 range when they appear on the market.
A great reference for identifying flasks is The Powder Flask Book by Ray Riling. It was published in 1953 and while it does not give values (they would be way outdated anyway) it is a fantastic reference to identify the type of flask you have.