Cheapest i can find looks to be around £60 on Ebay: Microsoft Windows XP Professional 64 bit X64 AS PICTURED FULL VERSION | eBay How come you want 64 bit Windows XP anyway? you would be far better off with something more up to date.
Windows XP in 2014 is going to be officially discontinued so imo, you shouldn't buy this at this point which is really late. However, if you don't mind using an OS without support, that's up to you.
There's an auction currently running with 3 days left if you still need it, currently at £10.50 plus p&p, Microsoft Windows Xp Professional x64 Edition | eBay, item is listed as new.