I am looking for a cost estimate for a manned (return) mission to Mars. There is a similar question asking for the cost of the Mars One proposalcost of the Mars One proposal, but I would be happy with a cost estimate for any type of human mission to mars. I need something substantiated and ideally citable, e.g. from someone like NASA. The Wikipedia article on this says:
The cost of sending people to Mars has been the main obstacle of any mission. Estimates of cost have ranged from \$6 billion to \$500 billion for various crewed programs.
There are sources given in the article, but I am not sure how substantiated they are.