Hi there! Been there and done that. I used to be a poor grad student and spent ca$h on the certification. That was about 5 yrs back.
In my experience the SCJP is one of the nice to have things and not a must have, especially if you have a Master's in CS and have meritorious record in your bachelor's and master's.
When you make it to the interview, the company that's looking to hire you will definitely do a lot of their own
testing so the SCJP isn't all that important. One thing's for certain: no one will hire you based on the SCJP; it's just one of a large number of factors that determine whether or not you get the job.
In my personal experience I haven't found it to be a useful thing to have.
Others experience might differ and you could end up with a totally different advice.
In short, it's a nice to have but it may or may not enhance your chance at getting a job.
If you're short on funds (as befits a grad student), I'd say go ahead and hold onto that dough but do make sure that you arm yourself with the knowledge needed to crack the interviews.
Good luck!
Sashi