Student discounts are usually the cheapest you'll get software... but prices vary school to school. I know that some colleges have XP Pro for $5 or so, while my school charges $35 I believe... it all depends on what deal your school has cut with the company.
You can try to search online for schools that have it cheaper... maybe you can get it without actually being enrolled in that school (tho that usually isn't the case).
You're probably going to wind up paying the regualar student price..... or um..... get it from Mexico
