Hello again,
I have found 3 events in the open sky challenge with consistent payouts to other series. Here they are:
(All payouts are basic and can be altered by friend bonuses, daily bonuses, etc.)
Group 20, Melbourne- approx. $20 000
You should always start with a Melbourne race at the beginning of farming. This lets you play 2 races instead of 1, as racing at Melbourne with any amount of laps will not give you a critical car condition. Fortunately, the Melbourne cup in this series gives out a reasonable amount.
Group 20, Spa- approx. $30 000
This should be your main farming event in this series, however not the first race of the day.
Group 19- Silverstone- approx. $25 000
This event gives out a reasonable payout.
Well let's just hope that your friends are playing these events.
Hope this helps!