If you don't necessarily want e-ink, the Pocketbook IQ is about $150. It's a new device, so I don't know too much about it yet, but it might be worth checking out.
For PDFs (I'm also a grad student, so I imagine that you are drowning in PDFs) people seem to be more satisfied with Sony products. The PRS-600 (last year's Sony model) and the PRS-350 (on sale this week, for about $150) fit in your price range, but the 600 is the older model with a worse screen. The PRS-350 has a 5 inch screen, but it's the new pearl screen, with better contrast. You can use reflow on PDFs and pan and zoom on the 350 and the 600. A Kindle wi-fi also fits your range. Here is a site that offers excellent reviews of these devices, including videos. I hope this helps a bit.
http://www.the-ebook-reader.com/pocketbook.html