A-body Disk/Drum masters for '67-70 NOS or NORS, yes can be expensive. I think I paid 200 and now they are asking 300. That's from Kanter DBA as OldCarparts? on ebay. Drum/Drum are not nearly so expensive last I looked.
In fact I have a used drum master I'm going to give the guy with the '68 Barrcuda. I've just been too busy to meet up with him. Someone put a later disk/drum with booster on his all drum car. Nuts.