Windows XP is not for sale nowadays. After the introduction of Windows Vista, Microsoft corporation stopped selling XP. Now the avilable operating system is Windows Vista which can be upgraded free of cost to Windows Seven after its release.
Microsoft Windows XP cost between 75 and 100 dollars, all depending on what version you buy. The home edition which is good for the average user is around 75 dollars.
Windows costs $117. 49 for Windows XP Pro Service Pack 3 and $90 for Windows XP Home. For my money I would buy XP pro. You get a lot more features and it doesn't cost too much more.