The majority of that cost comes from all of the shuttle flights required to put the thing together. You don't have to fly to-and-from Mars 40 different times. Another huge percentage of the cost comes from general operations work. Installing and supporting scientific instruments, EVAs, ground control, etc, etc. Most of those costs don't exist for a one way, no visitors allowed trip. The cost is also vastly inflated just by virtue of being a product of the US government. From what I can find, Mir cost just $4.2 billion over its ten year lifetime.
Anyway, I'm not at all convinced that the proposed project could be done for 6 billion, but I don't think it would be anywhere near $150 billion. It's not really fair to say "low earth cost us x, so Mars would cost us 2x!" There are a lot of different factors involved.
The majority of the cost is putting items into orbit (mass) and then assembling the mass into a usable form. How are we not going to have the same problem with any mars trip?
Mir was also almost 1/4 the mass of the ISS. Thus you are talking about 4 times the cost to make it the same size as the ISS.
It takes a lot of Mass to support humans in space. You would be putting far more mass into space to get people to mars than supporting a space station, remember you need to
1) get a crew onto mars
2) giving them enough resources to survive indefinitely without resupplies
3) give them enough resources so they can survive when something goes wrong for they are 8 minutes away from humanity
