Buying downloadable content on Microsoft's Xbox Live service requires a little extra basic math in order to convert your real currency into the company's own virtual currency, Microsoft Points.  While competitors such as Sony, Nintendo, and Apple charge actual money for DLC, Microsoft has continued to use its own Points system in which 400P equates to $5 (Nintendo's Wii and DSiWare still use the company's own Nintendo Points that at least have a 100P:$1 conversion rate, but the 3DS eShop shows a transition into using actual dollars and cents (or whatever you local currency is)).  It's not possible to buy, say, $2 worth of Points at a shot, and all of this Points stuff is meant to semi-trick you into spending more money than you think you are.  All of that mess may be coming to an end this year, however, as there's a rumor going around that Microsoft is looking to retire its Points in favor of real money.  Inside Mobile Apps has the story.  Here's the relevant bit:

By the end of 2012, all transactions will be based on the region set on the purchasing account and real money will be used to purchase all Windows Phone content. The move puts the Windows Phone Marketplace in line with the purchasing practices used in the App Store and the Android Market.

When asked Microsoft declined to provide further information, with a Microsoft spokesperson responding with “we do not comment on rumours or speculation.”

Mobile developers that have publishing agreements with Microsoft are being warned to plan their upcoming downloadable content (DLC) and in-app purchases in accordance with the change. Customers with existing Microsoft Points balances will have them converted into their local currency after the switch.

I've never had to deal with spending Microsoft Points, but I have bought Points cards for friends as gifts and always had to wonder just how many points I was getting for my money.  Some online retailers sell the cards for more than their actual value, meaning that if you don't know what you're doing, it's possible to spend $25 for 1600 Points which is worth only $20.  It's very easy to be overcharged when there's an extra layer of currency involved in transactions.  I'll be glad to see the Points system retired just for that reason alone.