What is the best way to farm coins?

In the backyard, right in the middle of the map there is a flag you can raise. Its like garden/graveyard ops except it is at your home. Doing this will not only grant you XP but will also give you some money from completing each wave. Remember as you go on, it will get harder but the rewards get better. It is also a great way to complete some daily missions that will often grant you stars. Stars are used to clear rubble where you can put statues at or you can find chest (that usually require 3 - 6 stars  to open). They will grant you money and/or possibly customization's or possibly a big pile of money for you to spend.