From The GPRO Wiki
If you don't cover 90% of the race distance you will receive only 50% of the race income according to the Race Winnings Table.
Example: If you finished in 40th position in Elite and didn't cover 90% of the race distance you will get only $5.500.000 for your race position.
This rule does not apply to Rookie groups.
GPRO Race Screen Anomalies
When watching the race, your car will always update to include each lap the leader completes. This can cause confusion if you have been smoking for a long period, and your race position actually shows you to be a number of laps behind.
Even though during the race it may appear that you dropped out after completing 48 of 50 laps, if you were actually 5 laps behind, then this is deducted from your lap count after the race - meaning you only actually completed 43 of 50 laps - and therefore did not reach 90% distance.
This is an "anomaly" (or "feature") recognised by the GPRO admins, and have it on their to-do list for the future.