How much does it cost to go to a movie?
Okay, now what's your answer if I told you that while the movie is taking place, you have to miss the final debate in the school board election, in a race where you're tied for first?
Clearly, the stuff you miss has a cost.
Freelancers are very good at having an innate sense of opportunity cost. They realize, for example, that taking on a friend at a discount might be very expensive if it means that other, better paying work is going to have to be turned down.
Now that just about everyone is in the business of selling their time in some form, it's important to be aware that even if something doesn't cost you cash out of your wallet, the opportunity cost is not only real, it's just as valuable. Not only does it cost money to say 'no', it costs money to say 'yes'.