NULLIF returns null if two values are equal, like if you did

NULLIF({custeventcustevent_initial_cost},0)

and {custeventcustevent_initial_cost} is 0, then it will return null. NVL checks to see if the value is already null and then replaces it with your second value NVL({custeventcustevent_initial_cost},0) so if {custeventcustevent_initial_cost} is null, then it will return 0, if not the value of {custeventcustevent_initial_cost} is returned