I've seen an analysis which said that the revenue source isn't going to space and back, but going really fast between 2 points on earth. The analysis basically said that with mail delivery you pay up to 5 times as much for the fastest delivery tier as you do fo the second fastest and that there are about 10% as many deliveries at the fastest tier as at the second fastest. So if you could use space ship 2 for an even faster tier and could get 10% of the current fastest market for 5x the price, then there should be enough mail to pay for up to 1 flight per day across the country.