A while ago I worked out based on the Inno set exhange rates (using diamonds as the carry currency) for goods (tech tree), coins (fp purchase), fp etc that a ration of 1fp to 5 goods per era was in the ball park. Don't ask me the logic as it is long forgotten and I'm not sure it was ever right
You need about 2400 OF goods so that would mean around 450-500fp?
On old worlds with lots of OF players swimming in goods waiting for the next era release the rate should be cheaper as they will have lots of goods doing nothing.
On new worlds where there are few OF players the rate is possibly higher as goods are in demand. This doesn't necessarily mean the number of fps should increase but that the carry goods to complete the trade should be better than BA/IA/EMA On N I've seen the junk goods traded being specified as FE... and the rate being reduced as FE goods are useful for OF players wanting to fight in FE...