It could be how Neotokyo did it- the meat of the soundtrack would ship with the mod (one disc’s worth), but you could only get the full OST if you bought the 2 disc CD. (Which I did, and it was completely worth the money. GET THAT SOUNDTRACK PRONTO)
It’s a good idea. Main OST with the game, B tracks, remixes, and cut tracks on disc 2 or whatever. Plus it could be formatted in a way where you could drop the OST’s tracks into BM’s music folder and it would play at parts of the game when appropriate. (e.g. each level would have ambient_generics that would play the default OST, but when you drop in the paid OST music you could get different cues that play either in the same spot or in places you didn’t get with the initial release.)
My God, I just developed a modding pricing model. What have I done?