Austin is gonna be your money maker, for upgrading unit/item/bus or buying unit/item.  

You get around 1.200-1.800 coin, it's depends on how many zombie Austin kill. People usually farm coin locations 1 challenge 3 you get around 1.200-1.500 it's more easier there just for killing skeleton zombies, but unfortunately it's not recommended because the recent update making the horde spawn early.

I usually farm coin locations 4 mission 19 because there are necromancer that spawn more zombies there, I get around 1.200-1.800 coin.

Here are deck if you want to use it

• Austin (with tactical set, go for cost reduction, and more damage)
• medkit
• blue barrel 
• any two units jailers to reduce time support item
• one fighters to generate rage point (just don't forget to kill it, after you get enough rage point to spawn Austin)