> I am quoted $398 ($441 tax-in) for a flight from LGA to SEA round-trip from Aug 25 to Sept 8. Hipmunk gives me tickets from $398 each way, so I'd be paying twice as much minimum.

This tripped me up the first time as well. That price is $398 round trip on Hipmonk. They show the best round trip price, not the best one way price. They say "from $398" to indicate that the best you can do is $398 if you pick the correct return flight.