I'm not an expert on this, or much of anything else, really, but here goes.
The value of antique powder flasks seems chiefly dependent on condition and rarity. There are a lot of old flasks out there, and of the ones I have seen, the leather-covered ones seem relatively less common. However, the flask you have there is in pretty sad shape. I don't think you would risk much by carefully attempting to repair it, and the experience will undoubtedly teach you something useful. If it were mine, I would try carefully disassembling it and cleaning up the individual metal parts.
The leather cover might be a problem. It looks to me as if it has shrunk, and I doubt there is any way to make it "grow" back to its original size, or to patch it neatly. The decision, I think, would be whether to leave it as-is, remove it altogether, or replace it with new leather of a similar type. Some sort of light-weight veg tan would be least likely to react with the metal.
I don't know the best way to deal with that hole in the side of the flask. There are some very talented metal fabricators here on this forum, and I hope some of them will offer some guidance.
Thanks for showing your flask!
Notchy Bob