Like any collectible, the range of value of WW II letters varies widely, depending on factors such as rarity, historical importance, and content.
Good content, is actually not that common, because letters were routinely censored, so many people didn't want to test the limits, because they might get in trouble, and also end up with huge hunks being cut out of their letters home.
If the writer of the letter was an officer it can also make the price of the letter go up. The higher the rank, the rarer the letter. Letters that were written before the soldier went overseas, usually will be priced lower than once they are in combat. Things like letterheads can also make the price of a letter go up. Really, anything that makes a letter stand out against the millions of other letters out there, will help increase it's value.
One strategy to use in collecting military correspondence, is to buy lots. A lot is a group of letters, and in most cases, buying a lot of letters will hold down the per letter cost. Some people even resell some of the letters, to make some of their money back. But reselling breaks up a group of letters, which can mean losing the ability to glean all of their historical data.