Hierarchical grid control strategies are an appropriate design concept for the coordination of future transmission system operator (TSO) and distribution system operator (DSO) interactions. Hierarchical approaches are based on the aggregation of decentralized ancillary service potentials, represented by converter-coupled, communicable active and reactive power flexibility providing units (FPU, e.g. wind turbines) at vertical TSO/DSO system interfaces. The resulting PQ-polygon made available by the DSO for a potential request of ancillary service flexibilities by the TSO is called feasible operation region (FOR). A monetarization of the FOR is necessary for the implementation as operational degree of freedom within TSO grid control. In the context of a local DSO and global TSO market, this article presents an approach for the monetarization of the FOR by a cost structure using metadata from population based aggregation methods. At the local DSO market free bids for the active and reactive power flexibilities by the FPUs stakeholders are assumed. Within the aggregation method multiple FPU flexibility polygons at a single bus are aggregated for a reduction of the search space dimensions. Thereby, the main contribution of the proposed method is the cost-optimal disaggregation of a flexibility demand to the single FPUs within the aggregated FPU by a mixed integer linear program. The approach can be simply adapted for the coordination of DSO/DSO-interactions regarding hierarchical multi-level grid control strategies.