The last time I checked, XP Pro with service pack 1 (OEM, can be bought with hardware only usually) was around $150 dollars USD. XP home was about $100 dollars.
OEM usually means "no fluff", like boxes or manuals. So an OEM version of Windows XP Pro probably doesn't come in the pretty box, but it will work just like the full retail version. Make certain that you get the FULL version, and not an upgrade.
Is Media Center out yet? If it is, I'm betting it will be more than the home version, but probably not as much as Professional.
Retail wise, Professional (full) is about $250 USD or slightly more and Home (full) should be around $150 USD and up.