Well if every one on a flight was to tip 10% of their ticket cost the math goes like this.

Based on a cheap ticket of $150
10%=15 bucks per passenger
$15 x 100 people on the flight= $1500
$1500 x 3 legs= $4500
$4500 / 2 pilots= $2250
$2250 x 3 day trip=$6750
$6750 x 4 trips a month= $27,000 a month
$27,000 x 12 months= $324,000 a year.

Not bad. Start tipping.