If you are looking for a cheap, legit copy, the absolute lowest I would suggest is finding illustrator 8. However you may run the risk of newer files not opening properly. They still share AI extensions, but the older version sometimes won't know how to correctly interpret the newer version's data. If it happens, most of the issues would be with typography and how it's handled. Not a big problem since you are primarily looking to manipulate and create line art.
For better compatibility AND functionality I'd highly recommend trying to find version 10 or CS1. This is when I would consider illustrator ALMOST tolerable.
Regarding student versions...
Most of the student version licenses I've read say something along the lines of... for attending student's educational purposes only and can't be used to produce artwork for commercial use. Other than the legal agreements they are identical to the full versions.
Still I wouldn't even bother because unless you are really a student it's not much above just getting a version off a friend or the net. At that rate, you might as well invest any money saved into buying a tutorial book if you want to learn properly. At the very least you can make it an official Adobe one if you start to feel guilty about how you acquired the software...
-baker
(I'm not a big illustrator fan at all, I've just had to deal with it a lot)