Firstly, apartments can only be regioned on island plots. Also, apartments need to enough space to fit the basic needs for a player: crafting table, furnaces, multiple chests, 'furniture' and other things. Without everything being too closely packed, that would need to be at least 6 by 6 or more. Also, apartments are required to have a hallway or room to get to the door, and this needs to be at least 2 wide and be a bock or a few blocks long at least. For a smaller apartment, 6 by 6ish, I wouldn't charge more then $250 as that is the recommended amount new players go to. Any rooms more expensive then $250 are not as likely to get rented and are not looked at by newbies, but perhaps by older players.