There was an interesting thread on the topic 1 year ago : http://www.reddit.com/r/AskHistorians/comments/14gdcm/how_much_in_modern_dollars_did_it_cost_for/

To summarise, it appears that estimating the cost of Columbus' voyage in current dollars is not trivial, however it seems that it would have been much cheaper than going to Mars today would be. Everything needed was already available, it was just a matter of trying a new route.