Like Slamfire said, condition is key. If someone tried to sharpen it on a bench grinder, cut wood, or otherwise do damage, the value will decrease. The condition of the various parts all count too. One of the mass-produced officer's swords from WWII might be worth 300-500 dollars if it is in really good condition.
The real determination of value is the blade. While tens of thousands of blades and complete swords were stamped out in factories during the war, some Japanese soldiers (most likely officers) may have carried "family" blades into battle. These may have been reappointed with military handles, but the original blade could be worth a considerable sum - to the right buyer. Most likely that buyer would be the decendents of that same family in present-day Japan. Some of these blades may be several hundred years old.
The likelihood that you have one of these blades is very slim, but not impossible. Ideally, you should carefully remove the handle and guard to expose the tang of the naked blade. Any inscription embedded in the tang will help identify the sword maker. You will likely need a expert in Samurai swords to provide an accurate translation and history.
If it is one of the mass-produced officer's swords, you could get a few bucks for it from a collector. If by chance you have one of the historical blades, you should (in my opinion) only consider returning it to the Japanese - for a price. It would be dishonorable for you to just give it back, esp. if it was lost in battle. If it is a blade that may have been passed down in a family for many generations, I would return it to them for whatever price they offered. This would be the honorable thing to do. If the family happens to be Suzuki, Toshiba, or something similar you may do quite well.
Of course, right now the Japanese have other things to worry about. Have the blade identified, do some research, and then make a decision.