Nobody really knows what the ultimate cost would be.  However, consider the fact that the STS was supposedly going to be 10 times cheaper than expendable rockets and ended up costing 10 times as much and that the $100 billion ISS was originally budgeted at $8 billion.

IMO, a trip to Mars will eventually cost in the neighborhood of $1 trillion dollars when all is said and done.

Dave Mitsky  

http://www.space-travel.com/reports/The_Bush_Space_Initiative_Fiscal_Nightmare_or_Fiscal_Nightmare.html

http://www.space.com/news/nasa_cost_040111.html