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.