Depends on what you mean by cheap. I am not a brass expert, but here are my thoughts:
1) There is a bunch of older brass on the market as owners pass away and families sell collections. This may create a bit of a market "glut"
2) A lot of high-quality high-detail plastic or die-cast models have been released over the past 15 years. Prior to then, the only high-detailed steam available was brass. Now brass faces the competition from these less-expensive plastic models, and the prices have come down.
3) Places like ebay make it easier to buy and sell brass. The market has become global, and the increased competition and awareness may affect prices. Generally, this should raise prices, but it also equilibrates the prices to what people are willing to pay. Pre-ebay, a person's options to where a person could buy brass from were limited because of limited knowledge. Sellers could dictate prices based upon how "rare" they thought something was. If someone wanted a particular model, they may not have many options but to pay the seller's fee. It's like having only one gas station in town. If one wants gas, that person must pay the piper. If there are more than one gas station, one can choose whom where the purchase is made.
Kevin