Housing isn't really a "cost" concept. You need sufficient housing to enable your city's population to keep growing. Farms (and pastures, plantations, etc.) all provide housing for your city, whether or not you have any citizens working those farm (or pasture, plantation, etc.) tiles. So, if builders had unlimited charges, it would be perfectly rational to spam housing-providing improvements (farms, pastures, etc.) everywhere. Of course, builders do not have unlimited charges, so you have to be selective about their use -- am I better off improving (or harvesting) that copper tile, or dropping a farm on that other tile to increase my housing (even though I don't have any plans to work either tile right now)?