There's straw paper--paper made from straw or grass stems, as per here: http://www.cs.uiowa.edu/~jones/prairiepaper.html -- I suspect it would be the poorest quality paper of the 3, but also the cheapest and easiest to make.
There's bark paper--paper made from the inner layers of bark. Something like this: http://en.wikipedia.org/wiki/Rice_paper#Mulberry_paper -- I suspect it would be the hardest to make, but better quality than straw paper, and with cheaper raw materials than rag paper.
And there's rag paper--paper made from old cloth. It would probably be the best quality, but also have the most expensive, or at least the most inconsistently available, raw materials, as people wore clothes until they pretty much couldn't patch them up any more, in those days... so, it'd probably be the most expensive.
And, then, of course, there's parchment and vellum (any difference besides source animal?), and more makeshift materials like bark, or temporary surfaces like slate.
Am I getting any of this horribly, horribly wrong?...
edit: part of what I'm going for is to have my scholar differentiate between the "good stuff", used for proper books (some printed, some hand-written), and the cheap stuff used for keeping records that aren't expected to be needed for more than a few years. It doesn't necessarily need to exactly match actual history, just be plausible with preindustrial technology and materials
But it sounds like the bark paper wouldn't be likely, if there was rag paper; so, just straw paper and rag paper, then, plus whatever grades and qualities of parchment.
