When I tell people that I collect WWI letters, usually the first question they ask is....How much are they worth?
That is a difficult question to answer. There are so many things that have to be considered when trying to determine the value of a WWI letter. Here are just a few things than can affect the value.
The rank of the person who wrote the letter....The letters of an officer, will usually be more valueable than the letters of a private in the trenches.
Was the writer famous? If the writer is someone who later became famous the value could very very high.
What kind of duty did he serve. If he was an aviator, his letters could be very valuable, but if he worked in saniation in a U.S. camp, and never saw France, the value wouldn't be very high.
Content....Does the letter have good content, such as mention of certain battles, death of a friend, being wounded, or anything specific, the value goes up. *Note* Letters were censored during the war, so good content is hard to find.
Condition....Letters from World War One, are now almost 100 years old, so some have been taken care of over the years, and some have not.
Letterheads....Does the letter have an unusual letterhead? If so, the value goes up.
Overall rarity.....If this letter is rare for any reason, the value goes up.