Assume you're going to make your game fulltime on your own.
How much do you expect to spend on food? lodging? etc
If this is your only job (working on your game) you would need to pay yourself a salary if only to survive the development.

Yep that pretty much sums it up. Even the indie needs to eat and sleep in a warm place. So therefore any game will need money in order to be produced. AAA level programmers also need to pay their bills and so do you Mankiw a famous expert in economics writes in his book that "The Cost of Something is What You Give Up to Get It." That means the time you spend working on your game is what you give up as that time could be spend working for someone else and earn you a salary. Hence you earn less due to the fact you do not earn money before the game has been sold.

Case 1: you work 40 hours at a firm and earn a salary.

Case 2: you work 20 hours at a firm and work 20 hours on your own game.

In case 2 you lost halve the income(at least in hours as tax will affect this outcome too).

In case 1 you might be too tired after work to work on your game or perhaps not but then you might also have a family and there if you work on your game in your spare time you give up the time you could have spent together with your family.

The lesson here is that "People Face Tradeoffs." and nothing is really for free as all actions will make you give up something.

Therefore it cost money/time/resources to produce a game of any size no matter if it is a AAA title or an indie title.


Also have a look at this link: http://www.slembeck.ch/principles.html