The operating cost per seat mile for the 736 is related to its weight and capability.
The 736 and A318 were designed for the days when service to every airport in the Northeast from LAX
, and vice versa to all the regional California airports from EWR
, and BOS
, was going to be viable due to an unprecedented economy.
Now that time is long since past... and we're flying RJs and lighter planes to our hubs and putting people on less frequencies of larger airplanes again.
Also... the 318s operating cost is a touch better than the 736s... the 736 and 73G have equal trip costs, but the 73G gives you more seats to make more profit.
We'll see how the 318 does for HP
, and AF