Using data from http://css.umich.edu/factsheets/carbon-footprint-factsheet, CO2 generated averages 0.39 lbs / mile per person..
2422 mile flight from NYC to Seattle, 4844 miles round trip, = 1,889.2 lbs CO2 per person.
At $0.021 per pound CO2, round trip cost = $39.67 to offset