  We consider priced timed Petri nets, i.e., unbounded Petri nets where each
token carries a real-valued clock. Transition arcs are labeled with time
intervals, which specify constraints on the ages of tokens. Furthermore, our
cost model assigns token storage costs per time unit to places, and firing
costs to transitions. This general model strictly subsumes both priced timed
automata and unbounded priced Petri nets. We study the cost of computations
that reach a given control-state. In general, a computation with minimal cost
may not exist, due to strict inequalities in the time constraints. However, we
show that the infimum of the costs to reach a given control-state is computable
in the case where all place and transition costs are non-negative. On the other
hand, if negative costs are allowed, then the question whether a given
control-state is reachable with zero overall cost becomes undecidable. In fact,
this negative result holds even in the simpler case of discrete time (i.e.,
integer-valued clocks).
