Facebook Monetization Principles
Deeply integrate monetization features into your game’s design, so they’ll be apparent to the player from the start, alongside all the game’s other main features. Make buying virtual goods part of the first-time play experience, so players know how to spend, and make the spending experience enjoyable. Price your goods (generally) high to start and offer discounts as needed. Functional enhancements tend to sell well, decorations not (usually) so much.
See Chapter 4, “Facebook Game Design: Principles for Growth and Revenue,” for more information.