I don't need to read the Wikipedia page on economics, I've read and used several textbooks. Although if you want to look at some relevant Wikipedia articles, follow the link to
http://en.wikipedia.org/wiki/Rational_choice_theory Economics, well at least microeconomics, which is what applies to this situation, is the study of human choice in markets. This does, as you stated, apply to pretty much everything, in terms of situations in the human record. I'm not going to regurgitate everything I know, but let me just show one example:
You are at a bar and want to buy drinks. You drink 5 drinks before leaving. Why? Each drink costs the same amount. You get a lot of benefit from the first few, but less and less benefit as you keep drinking, and eventually, you get no more benefit, because if you drank anymore (the 6th drink), you would not feel well enough to warrant paying for it, so you stop.
Now imagine instead of paying for drinks, you had to do magic tricks for them. It takes time and money to prepare each trick, and you have to do a new trick for each drink. Some people will value their time too highly to prepare tricks, and thus will not drink any. Others will prepare 4 tricks for 4 drinks, after which the 5 drink is not worth the time and money spent preparing the 5th trick, because they don't benefit enough to warrant the cost.
Bringing it back on topic, having children has many costs: emotional pain, childbirth pain, costs to support them as they grow up, strain on relationships and work, etc. Children also provide benefit: they allow you to live vicariously into the future, provide love and excitement, etc. These costs and benefits vary on an individual basis, and thus some will have children because the benefits outweigh the costs, and some will not, because the costs outweigh the benefits.