Would I get reported if....

A player is welcome to report any individual he or she believes may be behaving questionably, Meklo. So, yes, you could indeed be reported if you neglected to provide payment for an instance run-through.

Now, whether or not that neglect constitutes a violation would be absolutely dependent on the contributing factors and the agreement(s), if any, made. In most situations, this would simply be caveat emptor; however, even if you're a betting Troll, I would sincerely discourage you from reneging on financial promises made to other players. Not only could this jeopardize your in-game reputation, but it could dissuade your realm mates from buying or selling anything from you in the future.

Treat your fellow players with respect, Meklo. After all, the world of Warcraft is a shared one. :)