"OP has not provided his route, but with the route, there may well be a clear economic reason for the pricing he is seeing."
so using sfo-icn as the base fare (~ $600)
I quickly ran thru a number of cities on west coast, routing thru sfo to icn...each of these cities cost +/- $200 for a one way eco fare to sfo.
i guess predictable:
flights starting out of LAX/SEA were ~ $700 to get to ICN
flights out of Boise/SLC/Sacramento/PDX/Eugene were $1200-$1400, even though individual legs priced out to ~$750.
So i guess i shouldn't be surprised that smaller market venues get the thru ticket priced high, but a quick search of competing fares certainly shows ample capacity for someone in PDX or SMF (and to a lesser extent SLC) to find a $700-800 fare on a competing airline