I saw a US War of 1812 shako offered at a local show for around the same prices as Luke shared. Such things are truly not cheap. My damaged French shako plate from the battle of Bautzen was $1,100 and I was only able to get it through a trade. The Napoleonic uniforms in my study are all reenactor or stage props. I know I could never afford a real one. But they work fine for me as I like the red uniforms on display. My Welsh Guards and Canadian 48th Highlanders doublet are real, probably from the 60's I am guessing.
The two helmets on top of the bookcases are a French Dragoon, 1879, and the other is a Belgian Cuirassier 1850 helmet. The busby is 11th Hussars 1880's and the French shako is an 1840's officers. I got these items from selling or trading off my airborne collection. I wanted them because they give me the look I want.
Bottom line if it's a real round hat for the R. Marines of the Napoleonic time period I can see it being very big bucks. I also recommend when looking at leather headgear checking it closely as it often distorts it's shape from drying out, and can be badly cracked.
Good luck,
Terry