I built houses that pay well. Brown houses are good if you collect whenever you can, but blue houses allow you rest between collecting. It may take a week or more to buy the houses and land and build, but in the long run you will get everything back and more. I have over 100 million game cash now, and I own all non-premium houses and items. I just finished Burn's Money Mountain which costs about 11 million or so. I have tons of blue houses, and I try to collect 3 times a day (although sometimes it is just 2). One day I will bulldoze those blue houses, but I like how they give me XP to finish the latest stage over and over collecting 3 donuts multiple times.