This site has a guide of currect prices. It does not cover everything ever made, but is pretty broad on both old and new brass.
http://www.brasstrains.com/brassguide
I find ebay alone to be misleading. The first time an item comes up it may command a high price because 2 or 3 people want it. Then a few weeks later a second one will come up and get no bids or very low ones.
I think a better indicator is to attend a few O Scale shows and study the prices for what you are interested in. Generally the prices from show to show tend to be consistent. They do go down or up as time goes on based on current market and popularity.
