This morning’s Xbox Live update finally converts all Microsoft Points (“space bucks”) to the real currency of the user’s particular region. The conversion in the United States appears to be in-line with the standard point conversion rate of $1 = 80 MS points.
The system appears to round up to the nearest quarter dollar in real currency during the update. For example, I had 3630 points, which converted would be $45.38. It was rounded up to $45.50 in the transition.