you are likely to find Apartment in/at/on Budget. Budget is related to Monthly. Monthly is related to Mortgage. 
Complex is a synonym of Simple. Simple is the opposite of Subtle. Subtle is the opposite of Obvious. 
Apartment --AtLocation--> Budget <--RelatedTo--> Monthly <--RelatedTo--> Mortgage 
Complex <--Synonym--> Simple <--Antonym--> Subtle <--Antonym--> Obvious 