If you want a general purpose, scratchpad, midi sound module then take a look at the $60 Midiplus MiniEngine sound module
+++ Amazon +++ It is general midi compatible so it will work with Band-in-a-Box pretty much right out of the box. The S-Engine USB sound module costs $97 but has more connections
+++ 2nd Amazon +++ Both use the E-MU 8030 based sound engine with 64 voice polyphony.