So best routes are around the 27kM section.(2.2x increase over base cost versus 3x at 47km)
You're right.
The formula is =IF(Distance<60,175*COS((Distance*PI()/60)-PI())+175,320+Distance/2)
so for distance = 1...50 km, bonus = 175*COS((Distance*PI()/60)-PI())+175
If you differentiate it, you'll get a function with the maximum at about 29-30 km. That's where the trade is the most profitable.
The corresponding bonus will be 165-175%.
Nerds ftw!