I'm a longtime iPad user but I have one of the cheaper Amazon Fire tablets (whatever was the 7-inch model that sold for $30 for last year's Black Friday). I mean, it's a tablet, but it's not very fun to use compared to my iPad/iPhone because of how clunky the multitouch is (among other things). And the screen isn't high res -- under 200 ppi IIRC. Even the new Kindle Fire HD 10-inch model maxes out at just 224 ppi.
The ebook reader I currently own is the Kindle Voyage, which apparently has 300 ppi on a 6 inch screen at a cost of $199. The Kindle that I would recommend is the Paperwhite, which is just $90, while also having the same screen quality as the Voyage.
(I owned the Paperwhite before getting the Voyage. I don't have any complaints about the Voyage but I honestly could not tell you why it's any better than I remember the Paperwhite being).
So if PPI is a large factor in cost of screen, it turns out that the Kindle e-readers aren't that much out of whack compared to the Amazon Fire HD tablets. Yes, it's just black and white, but for most books, that's all you need. And it is very legible black-and-white. I have an iPad Pro but I do almost all of my e-reading on the Kindle.
If you're a student, one thing I might worry about is whether the Kindles and their small screen can handle all of the textbooks that you might use. For books where layout is important, the small Kindle can be kind of a pain. And some books (purchased on Amazon) will refuse to open on the Kindle, so I have to view them on my iPad. I imagine the 10-inch version of the Kindle Fire would handle full=size books, but that is me just guessing.
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You probably have seen these links, but just in case:
https://thewirecutter.com/reviews/amazon-kindle-is-the-best-...