The cost of a perfumery ingredient can depend on a number of factors, including its rarity, its production method, and its perceived value. Some of the key synthetic perfumery ingredients include:

Hedione: This is a synthetic molecule that is used to mimic the scent of jasmine, and is often used in floral fragrances. Hedione is considered one of the most expensive synthetic perfumery ingredients due to its complex molecular structure, which makes it difficult to produce.
ISO E Super: This is a synthetic molecule that is used to enhance the scent of other fragrances and to provide a long-lasting and diffusion-resistant effect. ISO E Super is considered one of the most expensive synthetic perfumery ingredients due to its unique properties, which make it a highly sought-after ingredient in perfumery.
Ambroxan: This is a synthetic molecule that is used to mimic the scent of amber, and is often used in woody and oriental fragrances. Ambroxan is considered one of the most expensive synthetic perfumery ingredients due to its high cost of production, which makes it a rare and valuable ingredient in perfumery.

These are just a few examples of important synthetic perfumery ingredients, and the cost of a specific ingredient can vary depending on the market and other factors. These ingredients are highly valued by perfumers for their ability to enhance and improve the scent of a fragrance, and are often used in high-end perfumes.