Great question. A recent report published by Syncapse Corp, analyzed (a) product spending, (b) loyalty, (c) likelihood to recommend, (d) brand affinity, (e) media value and (f) acquisition cost. In summary, they found:

1. Fans spend an additional $71.84 on products for which they are fans compared to those who are not fans;
2. Fans are 28% more likely than non-fans to continue using the brand;
3. Fans are 41% more likely than non-fans to recommend a fanned product to their friends.

In their study, Syncapse found that the average fan value was $136.38.

You can read their entire report here.