I've discussed the demand for ideas. What about the supply? (I'm ignoring beliefs for now.)
For many ideas, which are carried in books, on CDs, in movies, and the like, the supply works just as it does with any other material good. As the market price goes up, so does the amount supplied. The everyday upward-sloping supply curve for material goods applies equally to these ideas — because they are material goods. (Some advertising also works in this manner, with advertisers targeting those who are interested in the information they supply, and then adding the advertising cost onto the price of the good when a person buys it. In effect, the supply increases in response to the "price", that is, the value consumers are willing to pay for it.)
Ideas supplied outside of traditional markets are more problematic. What determines the information brought up in, say, everyday discussions?
The key to understanding the supply of these ideas in these situations is the realization that many social processes, though money is not explicitly involved, operate as markets.
There are 6 billion people on Earth, and you can only be in close relationships with a handful of them. The upper limit to the number of people a person can keep in touch with is probably a few hundred. Somehow, individuals must decide who to befriend and who to ignore. Thus friendship involves the allocation of scarce resources. Whether the reasoning behind the relationship is described best by social exchange theory, Gary Becker's rotten kid theorem, or something entirely different, the result is a market.
Let's say each person is looking for the most rewarding friendship he can get, without searching for too long. Since the value of a friendship is subjective, each individual is going to be looking for somewhat different qualities in a friend.
But there's not enough of each person to satisfy the friendship desires of everyone who wants to be his friend (well, maybe for some people— but wannabe friends still have to compete with other activities, like jobs and television). So a sorting process occurs, each person looking for the best friends he can get that will take him.
One of the things a person looks for in a friend is pleasing ideas. For example, a person who talks constantly about his cat probably won't get many friends outside of cat enthusiasts. People might avoid this person like the plague. A person with an unending supply of hilarious jokes, or who can give profitable stock market advice, will be more successful on the friendship market, other things being equal.
This gives friend-seekers an important incentive, which is the entire reason for jumping into this subject: the incentive to tailor their conversation to their friend's (or potential friend's) interests. By doing this, they can extend the range of possible friendships open to them. In other words, there is a sort of implicit price for information in the friendship market, and, by providing more valuable information, you can ask for a higher price. Presumably, people are aware of this, at some level, and respond accordingly. This means the supply curve looks something like this:
As the benefits of supplying information go up, so does the amount supplied.
Combine this with the demand curve from earlier, and the information market looks like this:
Economics and sociology: 1
Gary Becker, Altruism, Egoism, and Genetic Fitness: Economics and Sociobiology (from The Economic Approach to Human Behavior) -- the rotten kid version of friendship
Sergio Currarini, Matt O. Jackson, Paolo Pin, An Economic Model of Friendship: Homophily, Minorities and Segregation -- a market-oriented model of friendship
George Homans, Social Behavior: Its Elementary Forms -- the social exchange version of friendshp