This might be obvious to everyone else but, it took me a while to figure out.  I kept seeing the cost of upgrades and I couldn't figure out how you could get that much money.   Then I realized something.   Even if you are stuck on a level, play it through.  Even when you lose a level you keep the money for the cakes you sold.  Keep playing that level until you can buy an upgrade that will help you beat it.   I kept playing one level about three times and earned enough to buy the cookie maker.   This kept the customers happy and not only did I beat the level, I made expert. 

This only works with one older version of Cake Mania 3.  They've updated it, a few times, I think, and with the newer versions, that doesn't work.
 
If you want to download any of the new bonus levels, they make you download a newer version.
 
I upgraded mine to a newer version without knowing any better, so be careful not to download a newer version of the game if you want to keep this feature.

BUMP for candy

Thanks for the tip!