I seem to recall some rules about that within the Book of Vile Darkness, but I can't find my copy at hand, so I can't double check for you. Maybe that book just had additional, "vile" poisons.
i already looked through it and yeah it's just additional vile poisons. It says that you might be able to sell the bodies or glands at 1/6 the cost of a normal dose so that a specialist might render it, but it gives no instruction as to how that rendering process is done.
House ruling it.
So basically if you have a purple worm corpse it stands to reason that you still must make some investment of time and money to properly extract, preserve and refine the poison. But here I would say you need only make 1/2 the standard amount in GP with your weekly craft poison check to create one dose.