Ultrarunning and $$

Other than seriously overinvesting in running shoes, this sport of Ultrarunning is very affordable.  That is, unless you enter a lot of organized events, as entry fees tend to be high.  Not high in the sense of unreasonably costly--since all those aid stations and logictical support for 12-36 hours of elapsed event time do cost money--but high in the sense of writing a check for $150 or more, just to run a long race in the woods.

But my strategy is to only enter at most a couple of ultra races a year, and enjoy the rest of my ultrarunning as training runs in the backcountry...for free.