There are popular products using different licensing policies which are difficult to compare or even estimate over time. When you monitor 50 machines with NetCrunch it's always 50 licenses - no matter how many services, applications and parameters you monitor on each machine.
This type of licensing counts the maximum number of monitored elements, be it be node, volume or traffic point (VLAN, interface, port etc). To see the difference you can compare price of NetCrunch 50, which allows you to monitor 50 switches with 24 ports, to a 1200 element license price of our competitor.
This type of license is even harder to predict. You need to have good knowledge about monitoring something that needs one or more probes, and these kinds of licenses scale poorly.
For instance: monitoring PING responses on 50 nodes takes 50 licenses, but when you add additional service or start to monitor operating system, you will need an additional 50 licenses. Every single monitored object (volume, interface) needs an additional probe license.