Low prices for certain items on ebay reflect several factors.
One, the sale of an ebay auction item depends on more than one person bidding to increase the price; and if the right people arent tuned in at that particular time,the result can be a lower price. Others are just selling models to get rid of them and havent a financial need to get top dollar....
Two,condition of items often dictate the final value (used vs like new)..as well as the particular sellers reputation or lack thereof.
Three, it seems we are currently witnessing the great divide in pricing based on older/plain and used models (soft prices) vs. later run higher detail/quality models (steady or increasing prices). However, the softness in price of some of the earlier models (Max Gray and US Hobbies especially) is creating an excellent opportunity for anyone entering the hobby or who wants some of the best rugged quality and well engineered models available.
It seems the money (and price increases) are gravitating towards the high-end and rare models and older models in mint condition which were in short supply when made (i.e new models: Kohs,Car & Loco Shop,some Key-- and old models: Max Gray SP5000,L-131,T&P 2-10-4 etc.).
An exception is an interesting phenom which when Sunset or the like offers a copy of an existing top model (or a model is done in plastic of same),people tend to panic and dump the top models which causes a price drop. This is usually when the knowledgeable collectors buy. Often the prices for the top models go back up in time....this is currently playing out with some Key diesels which in a few cases are selling for 1/2 price.... read: opportunity....other Key diesels (not done in plastic etc.) are selling at premiums still.
In good condition, no matter which model, the price of new brass and quotes for same makes even $300 freight cars look cheap....try and get a brass hopper with Yoder type quality made in Korea today for less than $300 cost.....I just received quotes for a pickle car between $5-700....
In short,while some prices may seem high, they are relative bargains for what you are getting vs the cost to produce same in todays dollars. When the importers finally give up due to quality and price issues, the existing supply of models in good condition may increase significantly in price...
There are actually several guys,not necessarily model railroaders, who are collecting brass models (and other collectibles),not as an investment per se but as a store of value or tangible asset/inflationary hedge etc.....paper vs brass