Tablet PCs and UMPCs are great for reading. However, they are also full PCs so new ones don't quite fit the price that you're wanting. You might be able to find used ones in that price range, then download one of the several ebook readers.
Sony has a portable ebook reader. Have you looked at it? It's not a PC and would be just for ebooks, but something to consider:
SonyStyle.com | Sony® Portable Reader System