Craig,

I made a mistake in thinking that Max was specifying
two spacecraft, because he said "Pioneer 10-11". I now
think he only meant one, which would cut the cost a lot,
not least because of the reduction in DSN time. It might
even cut the total cost in half. Not that my estimate was
much better than order-of-magnitude to begin with.

On the other hand, I wonder if you noticed that Max
specified "to Neptune and *back*"?

I don't know how much cost getting back would add, but
it would have to be a lot. I assumed that getting back
means a flyby within a few million km of Earth. I'm sure
that getting back means a slower trip out, thus my
estimate of "about 20 years" for the mission.

-- Jeff, in Minneapolis