This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Apparently $1 through $9 are regex meta sequences for capturing groups. Your $50 was interpreted as the 5th capturing group followed by a 0. See the Javadocs of java.util.regex.Matcher, especially the appendReplacement mehod, for more info.