The cost is vastly in the multiple layers of redundancy built into the Airwave network - that and the network over-provisioning for 'emergency' situations.
On the operational (i.e. radio) side there aren't many opportunities for a single point of failure. Even on the internal side there is a significant amount of resilience (way above any commercial telecoms operator) such that they can actually provide accurate 'call' data to piece together a complex event. Whereas every few weeks it seems some area of a commercial telecoms provider's network disappears for a few days.
Even when the police had to bring in thousands of extra officers (from different regions) to London for the riots - they just selected the right talk group(s) and got on with it. The increase in traffic was substantial, and somewhat extended. I don't see that sort of 'spare' capacity being available from a telecoms operator, they can barely manage to send a text message at or about the New Year.