So,
6.8 billion...
Russian contract was 6 seats from 2014 to 2016 (with final return in 2017 the included) for 427 million $, making it about 20 million $ per year per seat.
If same calculation is valid for Boeing, going with ringpriest 20 million estimate and assuming no cost overruns that will be about 7 million $ per year.
6800 / (20 - 7) = 523 seat-years... divided by 6 seats actually requested by NASA from Russia, 87.
I guess i was way too optimistic with decade. 87 years, oh my... Unless traffic really picks up this change will not pay for itself in close to century...