Do not forget that the most popular Trench Art comes out of W.W.1 and next year will be the Centenary of the start of that conflict. Keep an eye on the prices therefore in the near future!
One area where prices appear to be gaining ground is medals. This might be something to do with the Afghan effect.
Item that you used to see often such as a simple brass letter opening knife still sell for a small amount, between £5 and £10.
Some things sell for high prices. Helmets and propellers can sell for about £800. Pickelhaubs, the fancy German helmets with a point on the top of them, sell for a great deal, even replica ones.
It is worth learning about because Trench Art is usually not very attractive. In fact it can look like junk and be quite depressing to look at. There is therefore still loads of it on flea markets and the like to buy that will only appeal to the ones in the know and you can take advantage of this because Ebay has a category devoted solely to Trench Art.
Most of it is easily posted and is ideal as something to sell on Ebay.
Go and have a look at it and buy a guide. I would get one on the history of it and also one that deals with the financial situation. It will never become very valuable because it is folk art but it is none the less a fascinating subject to study and collect because it is part of our historical record.