The cost isn't in paying someone to not use the orbit, it's that the busier a part of space gets, the more expensive it becomes to do collision avoidance and station keeping.
What makes this impossible to calculate is that there's an unknown exponential involved. The more satellites, the more collisions that need avoiding. And the higher the chance that one avoidance will create new future collisions.
At some point the space is simply so busy that collisions can no longer be avoided.
It’s really not impossible to calculate, particularly if you’re trying to cause damage.
The answer is it’s cheaper to shoot down individual satellites than try to create a localized cascade. Kessler cascades propagate too slowly, and degrade too quickly in low orbits, to be useful as a military tactic. In high orbit one could feasibly e.g. deny use of a geostationary band. But again, it’s cheaper to just shoot down each satellite.