It depends on how much you want to idealize things. If you add the assumption that all flights/trains/etc. are available every day, and the price never changes,

It depends on how much you want to idealize things. If you add the assumption that all flights/trains/etc. are available every day, and the price never changes, then yes. In reality, trying to traverse a weighted graph optimally with a bunch of constraints (arrival times, how long to stay in each place, preference for difference modes of transit, transit times) is a whole field of research in math/CS. Operations research [1] is the study of optimizing processes with many interrelated factors. 1. http://en.wikipedia.org/wiki/Operations_research http://en.wikipedia.org/wiki/Operations_research