Graham Jans' Blog - Microtransactions Under the Microscope. Thanks Mike Martinez.
As mentioned above, the value of real-money purchases is largely defined by the player's perspective within the game world. But additionally, the real-world value of items affects their perception within the game. The obvious case of this is that selling a top hat item for $1,000 will provide a kind of instant prestige for any player owning that item, even if it has no intrinsic value or significant aesthetic value. It's valuable because it's expensive.
There is a more subtle case with content that can be accessed both through real money and in-game effort. Take, for example earning a new Champion in League of Legends. On one hand, the paying player can say, "Woo, I payed $5 and saved myself 5 days of effort!" But the non-paying player can also say, "Woo, I earned this myself, and saved $5!" It actually gives an extrinsic value to the time the player is spending in the game.