We have quite a few of these style questions, which are only going to become more common.
Without fail, my first reaction is to want to answer that hitchhiking is the cheapest, but obviously, many people don't want to do that.
Currently in our [faq] we say not to ask what the cheapest [x,y] in place [X,Y] is - eg cheapest hotel in Hell, or cheapest flight from London to Miami.
I'm considering extending this to prevent 'cheapest way from x to y' questions. In a way it's like a shopping question - we should push for more questions like 'How to get from Mendoza to Santiago?". It should be up to the user, IMHO, to decide on the merits of the costs once they have the options presented.