By this stage you should have around $100,000 - $200,000. (I only had about $170,000 at this stage). The early game's focus was on improving our Police Force and levelling them up enough to be able to handle any call you want. You may start to think you won't have enough money by the end of the game - this is not true. At this point in the game is where the money goldmine kicks in. There are quite a few ways you can make money, and here are the notable ones:

The Mafia: If you are still cooperating with the Mafia, you will be able to sell your goods to the mafia and they will give you a percentage back. Say you send your officers to take down a drug lab - if done succesfully you will have drugs such as Heroin in your possession. You can take these to the Mafia to sell for a price. You will receive your money the next day. The amount of cash you get from these items depend on the items themselves, and can have a rather large range. You can sell a ficus plant for $5; or sell an assortment of automatic rifles for $15,000. It all varies on the goods you take. But be careful, as you will lose some of your reputation with City Hall if you do this everytime this option comes up, so be sure to take some items back to the Police Department. Furthermore, once you get the money from the Mafia, you have an option to share 50% with your staff - this is useful to keep a good standing with your staff members so they won't file reports against you (Will go into detail about this soon).

Solving Crimes: Solving crimes can be a tedious task, but can also provide you with a nice bonus in your wallet. Plus it's good to see that you don't have investigations pending. These investigations can give you a few thousand dollars each, and provide some decent experience points for your detectives. This isn't the best method to get money, but it's practical and legal enough.

Taking Down Gangs: This is by far the most effective method of gaining a decent amount of money. Although it does take time to take down a gang, the pay off is great. *Mid-Game Spoilers coming up* You can take down the Sands (if you side with them) through a rather long string of informants. Taking down their leader (Christopher Sands) will give you a fat paycheck of $200,000. Which will boost your money up to the $300,000 - $400,000 mark. The best part is that this is not the only gang you can take down - there are multiple gangs that are waiting to be destroyed. The payout will range from the size of the gang and their notoriety. The easiest gangs to take down (which will have about 2-3 informants, and 1 gang leader) will grant between $40,000 and $70,000 each.

Random Calls: The Mid-Game is also the best time to be taking on random calls for money from clients and customers. This will require you sending your officers to these calls. A lot of which are highly beneficial in more than just a financial standpoint. Some will grant your officers +100 experience, a better reputation with your Officers etc. Of course, some of these can do the opposite: You can lose your officers for the rest of the day, your officers can quit and your officers can die (only from one I've seen). In the mid to late game, you'll be called by some medical clinic to test a new virus on 4 officers. From my playthrough only one of my officers died, and the other 3 were fine. I also got $80,000 from this one call alone. So this is definitely recommended to take.

Smart Spending: Make sure that when you do spend money, that it's a wise investment. Using money through your associates is where the majority of your cash will go. This includes correcting mistakes, hiring new officers legally, firing officers legally, managing accounts, and many other things. Never use the private investigator to help you solve Investigations. $80,000 (I think) for a private detective is not worth it; if you're struggling with the investigations, just look it up on YouTube. Never purchase anything through the Church. It's useless and does absolutely nothing. The Price Range will be between $2,000 - $15,000 for absolutely nothing. As much as "praying for your officers" sounds cool, it's not. Avoid the Church at all costs - thankfully you can take down the church later on for Drug Trafficking and Production. (Which will give you a good $80,000 I think).