The average airplane flight emits about 0.4lb of CO2 per passenger mile. Compare that to a decent 20mpg car, which emits about 1lb of C02 per mile. Of course, if you carpool, you can divide that by the number of passengers. This confirms most calculations I have seen that show that flying gets about 70 mpg per passenger (source) However, because the effect of CO2 at that altitude is greater, a correction factor gives an effective mpg of 35mpg per passenger, still better than most cars, but not if you carpool. If your flight isn't completely booked, your share of the CO2 emitted would be even greater. In general, about 20lb CO2 are emitted per gallon of gas (source)
So, driving or flying 2,000 miles would emit 800 lb and 2,000 lb, respectively. But for two people, the figures for flying would become 1,600 lb, versus 2,000 lb driving. Let's just say that it's about a ton.
What would it take to offset these emissions?
To offset one ton carbon costs about $15. A number of companies will do this. "Afforestation of crop or pasture land is estimated to have the potential to sequester
between 2.2 and 9.5 metric tons of CO2 per acre per year. Reforestation is estimated at 1.1 to 7.7
metric tons of CO2 per acre per year." (source: Congressional Research Service) So a fraction of an acre could sequester the carbon from a cross-country airplane flight.
Interestingly, the cost to plant an acre of trees is $250-$2,000/acre (average: $500). According to this analysis, if afforestation (greatest potential to sequester carbon) cost $200/acre it could be profitable at a carbon price of $21/ton.