How does he make money to pay for things like food and Joan's stipend? It's stated that he doesn't get paid by the police department. We know he doesn't pay rent, but at some point, he says all the money he owns is $22,000, that can't last forever, especially when it's split between two people.
Note: I'm only midway through season 2 so if this is explained at some point then you have my apologies.
When they do non-police cases such as series 1 canon-ebersol (bankers), he is paid by them. Whilst he does not like taking adultery cases and etc. he takes them for money, if needed, also I think I remember a fund he gets a certain amount from a month, that is almost taken from him by his father. So a mix of those I would think!