Conferences aren't cheap. However, if you are a student be sure to read all the way to the end of the registration page where the student discounts are listed. Students make out like bandits... and that's a good thing.
My first Usenix was, um, well, it was a long time ago (most students today weren't born then). As a student I was able to get in for little money, I think my department chair chipped in a little, and I crashed on the couch of a local friend's parents.
That first Usenix conference changed my life. It was the first time I had real interaction with the Unix community and I learned more in those brief few days than I had in my past year of school. I also got to meet many of the open source gurus that I had only read about. The interesting thing about Usenix conferences is that the celebrities are available to talk with; they aren't roped off in some "green room" and unavailable.
I encourage all students to attend LISA. It helped me get my start, and it can really help you too.
See you there!