Nowadays not a very common drop. Costs have also changed since the early days - Now every conversion is free. That is, 0/pc.
The Greater Brie Alchemy Potion has the following conversions.
115/pc One potion converts 16 Swiss to 16 Brie.
120/pc One potion converts 12 Marble to 12 Brie.
125/pc One potion converts 8 Cheddar to 8 Brie.
Dropped by a Vampire Mouse at the Mousoleum.
Dropped by Abominable Snow mice and Elf mice during Christmas.