Once you get to upper levels, or if you are camping on a single level so that you can increase your cache of weapons and win fights easily, property income is likely going to be your largest source of income. Each lot you buy and building you place on it gives you a regenerating fee, the quickness of which depends on your selected class. There are a few ways to approach the property aspect so that you come out on top.
First off, this should be your main purchase point from the very beginning. Instead of spending your income from missions and what not on weapons right off the bat, you should begin acquiring property. A good rule to think about is to make sure that you have five pieces of property before you begin buying weapons. Since no one attacks in the first or second levels you should try and stay here for a while without weapons, or with minimum weapons. For the first couple levels try to only get the number of weapons that you need to complete missions, but then go ahead with more property buying. Do this until your property income is reasonable enough, then sit on a single level for a while until you bulk up your weapons and defensive items.
For every piece of a certain type of property you own the price will go up. This means that for every Shopping Plaza you have the price of the next one will be a little more. This also works with empty lots that are converted once a building is built on them. What this means is that you should buy them one at a time to build on because in this way the price will remain at its base for each purchase. This is a good way to save unnecessary money when doing the same purchase in a large volume.
Price Versus Pay Out
Take a look at the types of properties and the amount of money they give you. You will notice that the price differentials between how much they cost and what their pay out is on many properties is not worth it. For example, the Casino costs $50,000,000 for your first unit and pays out only $250,000. The Shopping Plaza does not cost anywhere near this, and pays out $100,000. It is best to find the most equitable properties on the cost-pay out dynamic and purchase them first. Once their prices go through the roof because of the number you own of them you can begin going toward the other properties.
Empty Lot Price
Also calculate in the price of the empty lot you must develop on when calculating the final decision. The Night Club gives you $150,000 for a reasonable purchase price, but the Sea Front lot it must be built on costs $4,000,000. Keep this as part of your calculation.