Let's see... I have +30 Presidential Suites, check. I have several discos and restaurants, check. Now to see my monthly income... [$5 per month] Okay, now we are getting somewhere!
There is this lady that wants to buy a knife for her son but says she only has 125 coins (the price is 135). So I suggest to her to buy a gift instead (450 coins). What does she do? She buys the gift full price! :D
Here is an hint to finish the game faster: First upgrade your ship with a few lasers and a magnet, and collect the coins almost as soon as you destroy a ship ('cause the longer you wait to collect, the less they are worth.)
For view of full elements visit:
http://ultimatewalkthrough.blogspot.com/2010/11/doodle-devil-walkthrough-for-all.html
It helped me a bit in finding the last one.