Money will be a major factor in your fight against the aliens, as with most things in the real world, there is little that you can't achieve with the judicious application of a huge pile of cash. The economics of your situation come down to a few major sources of income, and a somewhat larger number of costs.
There are three key ways in which you can accumulate more money for your organisation.
The first source of proper income, and your only one right at the start of the game is the initial Country Funding you have been given to get your organisation off the ground. This will get paid to you at the end of each month, and to retain – and increase – this funding you will need to keep your paymasters happy, either by keeping the UFOs off their front lawn, or by causing them to crash down on top it.
Country funding can remain an important revenue stream for most of the game, although towards the end you may well find yourself dreading the end of the month as the amount of funding you are getting dwindles compared to your growing monthly costs.
UFO Recovery missions
Although you will want to keep a fair amount of the items recovered from UFOs for your own use, there are many private buyers who are willing to pay out large amounts to get their hands on this new technology. As you progress towards your task of defeating the aliens, UFO Recovery Values tend to rise steadily, and are a very important revenue source. You do have to pay attention to not selling alien items that you will later want, as manufacturing them yourself is often significantly more expensive than the sale value of the item.
Sale of Manufactured goods
As well as being essential to produce some key pieces of technology that you have researched, your manufacturing teams can also be extremely profitable once they are up and running. It will take some seed capital, and some work from your eggheads, to get things going, but once you do it can be a very key part of your financial stability, as private buyers are also willing to purchase many of the items you will become able to produce.
Be wary of falling into the END OF THE MONTH trap when your Monthly Expenditures are greater than your Funding Council Income. Here's an example: Your manufacturing plants generate $30,000,000 a month, your Funding Countries income is $10,000,000, bringing in a total of $40,000,000. However, your monthly maintainence is $20,000,000. It would look as if you make a neat $20,000,000 a month, right? However, what happens if your cash balance at the last hour of the month is $1,000,000 and you have no items that you can sell? The new month, your starting cash will be -$9,000,000. This will greatly worry the Funding Nations, who will discontinue X-com if you are negative cash for another month. Also, perhaps more importantly, you will be unable to start any new manufacturing, and thus your entire manufacturing line is halted. So no $30,000,000 from manufacturing this month. How to deal with $10,000,000 income and $20,000,000 maintainence then? ... you had better hope the aliens come fast and furious to give you booty to sell!
This end of the month trap usually occurs when starting up a large scale manufacturing operation. Usually, there will be a huge surplus of money and spare alien guns, which can be sold, allowing construction of many manufacturing modules and hiring Engineers. Then more aliens come, and more tech is sold off, more engineers are hired. Eventually, the end of the month comes, and suddenly you realize that you have bitten off more than you can chew.
There a few different ways to spend all that money you are accumulating, some of which will be very important at certain parts of the game, and not so at others.
Both the creation of new bases, and the expansion of existing ones with new facilities makes up a fairly large proportion of your overall expenses, particular for the first part of the game.
Each base you build, and each facility within that base increases the monthly cost you will have to pay to cover your base maintenance costs. While not a huge chunk of your expenditures, it is a constant drain to be aware of. Don't build facilities that you don't need yet, however always remember to consider the long lead times that some facilities have.
All your base personnel – scientists, engineers and soldiers – and any Interceptors or Skyrangers you are renting will cost a fixed amount of money every month to stop them quitting your employ, or being repossessed. Don't these people know there's a war going on?
Equipment is usually not a major cost, simply because the technology available for you to purchase mostly is limited in application, and you rapidly become able to produce or acquire much better items for yourself. However, some items remain useful throughout the duration of the game, and keeping a steady stock of ammunition for the cheaper heavy weapons can be expensive.
Manufacturing Material Costs
While at certain times in the game manufacturing can be very profitable, it will also be essential to make items that have no resale value, such as the Avenger. Be well aware of the costs of the items you are making for your own use, to ensure they are worth the money you are spending on them. Also be aware of the exotic materials used up in the goods you are making, both from an opportunity cost standpoint (you could have sold those Alien Alloys instead of using them to make something, so they effectively increase the cost of anything that uses them up), and also particularly for Elerium-115, some materials can be very tough to replace when you have used them up.