I would say it depends on how well you are doing.... I charge nothing. For a mid-table team that has given 30K supporters.. I will charge if I do better next season.
I'm running a successful first season in Div 2.3, regular podiums, I make around 20-30k fans a race, and charge 100 for a subscription fee. It pumps up my income by around the 2.5mil or so each race. But for that to continue, i need to remain this successful.
So do we think the number of fans depends on the result, probably with a random element thrown in, then scaled by some sort of deviation from a magic number? Ie if the magic subs price was 500, at 500 or less you would get 100% of the potential fans. At €1000 you would get half, so on? I guess I could test this by lowering it and seeing what happens next race, assuming similar results.