From Desert Operations Wiki
Money in the beginning is a very important resource. You will use it to construct Buildings, Units and pay for the research. Units will always cost you every 10 minutes a fix cost. You should always make sure your tick remains positive. Money is the only resource that can have a negative number and put you in the red. You can make money from the Factories, Residential Houses or the Skyscrapers you build.
You can also earn money from the Bank by collecting interest from the savings. The money is paid out as available money and is not added to the money in the savings. The calculations for the interest depend on every server.