Buy is a synonym of Purchase. Purchase is related to Pay. Pay is related to Expense. 
Overlook is a type of Survey. Surveillance is derived from Survey. you are likely to find Surveillance in/at/on Casino. you are likely to find Security in/at/on Casino. 
Buy <--Synonym--> Purchase <--RelatedTo--> Pay <--RelatedTo--> Expense 
Overlook --IsA--> Survey <--DerivedFrom-- Surveillance --AtLocation--> Casino <--AtLocation-- Security 