If you would otherwise be doing anything with positive expected utility in that time, the opportunity cost is >$0.
The fact that the analysis can be carried out in monetary units (because we don't have a good direct measure of utility) doesn't mean that receiving money itself is the only source of utility that needs to be considered.
The fact that the analysis can be carried out in monetary units (because we don't have a good direct measure of utility) doesn't mean that receiving money itself is the only source of utility that needs to be considered.