Apart from the uncertainty about whether an agency will deliver a future reward at all, there is also the expenditure of resources in keeping track of and following up on a debt. If you have to keep track of a debt for a year, and take action to claim it at the end of that time, that could very well cost more than $50 worth of time and energy, particularly when you take into account the need to spend resources retaliating if the debtor fails to pay. I think it is much more plausible that people use ad hoc heuristics developed to deal with these issues, than that we actually use a hyperbolic discount function.