Ok, if you are talking about JUST Athearn sales then I'm confused about your statement. I know of several LHS where I can pick up Athearn BBS's and get 3-6 engines for $100 (their reduced retail) and 8-12 cars for another $100 (their reduce retail). I'm going to guess that they don't take a loss at those prices (they may not make anything either ... I don't know). If the cost from distributor to LHS is 40% of retail list then that's $500/mo in Athearn sales (at retail list).
Other posters have pointed out that the minimums from Horizon are for the entire Horizon line and YES I agree that you best be selling a lot more than that every month if you want to stay in business.