A trick I just recently learned is to use specialized rotating governors. Look for generals with cost reducing VnVs for infrastructure and units (usually one guy won't have both), and use them as temporary governors. For example, I have a guy who can build infrastructure at 30% off sticker price. I move him into a city and make sure he's the "official" governor (sometimes have to move another general out), then have him "buy" one or more improvements. I then move him on to another city and repeat the process for every city he can reach that turn. You know how queuing items removes money immediately from your treasury? That's why this works. Even after he moves on and another general takes control of the city, the cost saving stays. In fact, you can use another general with unit cost saving VnVs on the SAME turn and use him to buy units for multiple cities.

It can be something of a hassle to micromanage at this level, but the cost savings really add up - it's definitely an effective method for stretching out a tight budget and maximizing your $$.