Microsoft and other companies engage in a process called site licensing. Say your university upgrades 800 computers from Windows 2k to XP and from Office 2000 to Office 2003. They don't buy 800 boxed copies. And they don't want to have to buy five more copies for the next five computers they get. So they sign a site license which basically covers all of their operations based on estimates of how many seats there will be. This has gone on for many years.
In more recent years, MS and others started extending site licenses so that the institutions could add home use by their people onto their license for a nominal per user fee. This was a big perk for the universities and institutions, because they got to offer $10 copies of Windows XP to their users. It was a great perk to users, because they got software on the cheap, with liberal limitations (basically, that $10 copy is no good for resale, and sometimes cannot be used after one leaves the institution, although I think in MS's case, that is not true. But at $10, who cares? 😉 ).
That's the system under which others are getting Windows XP for $10 or $15. Whether it is available to you is purely based on wether U-Dub decided to negotiate for this with MS as part of their deal to use MS products on campus.
Go to your bookstore, and ask if home use site licenses for XP are available. If they are, they're probably substantially cheaper.
If they're not, your only real recourse is to write to your IT or administration and complain about it. 🙁
EDIT: Sorry, clearly spent too much time being verbose. 😉