The cost is exactly the same cost of calling a Blueprint function node.
The difference is within a Blueprint function graph you can have another tons of nodes in there and each of those nodes add up another “Kismet call” cost.
I can have over a thousand functions inside this node… The cost for a “Kismet call” is still going to be just 1. Everything within after that is C++ land.
Only the “Execute” function is the Kismet cost (Blueprint overhead)
It’s the only thing that the Kismet Compiler, well, executes.