Essentially every game which does microtransactions does it by implementing a dual currency system, with one free currency abundantly available if you have infinite time to play the game, and one currency which is only available through purchase (or very carefully monitored promotions). You sell the dear currency in odd-numbered lots (to exploit people's inability to do fourth grade math -- sad but broadly true of the industry), for example, 7 dragons eggs for $10, 15 dragons eggs (one free dragon egg!) for $20, etc. Even though this essentially enables you to price your Sword of Pwnage +1 at $1.42 (one dragon egg -- available ingame for one-click with no need to grab a credit card), you'll never actually send a $1.42 transaction to the processor.