You probably will not have to spend more than about $50 for a good loft. Lumber will be about $40 and the hardware will be about $10. I didnt really know what I was doing so I bought a plan. I bought one from these guys and it worked out pretty well:
http://www.campusloftbeds.com/loft-bed-plans-c-24.html
I also ended up adding a shelf to it like this:
http://www.kaboodle.com/reviews/bunkpal-bed-shelf-4 which came in pretty handy for the alarm clock.
Just make sure you tighten everything down really good. I didnt have very good tools and the bed was kind of wobbly near the end of the year.