Terrain doesn't matter - all other points do. Treasury levels play a role as well as do number of owned settlements. Some factions like Parthia loose the ability when they 'settle down'. Notification is a one time event.
Code:
; === Devastation Money Bonus ===
; Money bonus for devastating tiles on the map
; Aedui
monitor_event GeneralDevastatesTile FactionType f_aedui
and Treasury < 30000
and AgentType = named character
if not I_IsFactionAIControlled f_aedui
and I_EventCounter HE_DEVASTATE < 1 ; not been notified before
historic_event HE_DEVASTATE factions { f_aedui, }
end_if
if I_NumberOfSettlements f_aedui < 11 ; less then 10 settlements
add_money f_aedui 800
end_if
end_monitor
; Parthia
monitor_event GeneralDevastatesTile FactionType f_parthia
and Treasury < 30000
and AgentType = named character
if I_EventCounter ecParthia_Imperial > 0 ; settling reform has taken place
terminate_monitor
end_if
if not I_IsFactionAIControlled f_parthia
and I_EventCounter HE_DEVASTATE < 1 ; not been notified before
historic_event HE_DEVASTATE factions { f_parthia, }
end_if
if I_NumberOfSettlements f_parthia < 11 ; less then 10 settlements
add_money f_parthia 800
end_if
end_monitor