Value can be determined by demand, supply, desirability, habits.
With brilliants we can only have the first two parameters. The others will present themselfs after first deals.
So a person can make 6 brilliants a year. Barring GM's spoiling the game you can create only 6 gold coins a year. Cost of those 6 coins is ~300-400 credits with extra benefits.
So i'm at this point valuing a brilliant at 7gc. You are free to refuse or counter this is all good for determining the pain point.
Miq
Edit: i forgot the demand part. Every player wants a currency that can be obtained from in game activities so when a ratio is atleast somewhat set the demand will rise.