First, consider this chart (as reproduced by Matthew Yglesias):
Now examine Aaron Carroll's great rebuttal!
In general, I think Yglesias is correct that it is difficult to have any real reliability for 75 year cost projections (I wonder what the confidence limits are?). So much is likely to change over this period of time and the prioirites of the nation may be so different that it is completely unclear how helpful such an exercise will be. Not only do we have issues with technological and political change, but it would be odd if no future government altered policy priorities or if we could accurately guess economic growth over such a period.
37 minutes ago