In a nutshell you are right. Subs are a very expensive segment of the hobby. By and large you don't get much for say $500. but if you look around you can find bargains. Ebay is a great source, so are the classifieds on RCU. I just purchased a Sheerline Ohio class "Boomer" sub for about 1/2 it would cost to buy new. You did neglect to explain your definition of "large" and "cheap". Your definition may be different from mine so unless you put some numbers up it is hard to tell exactly what you are looking for.

Pete