The Kindle DX has demonstrated that a 9.7" EInk screen is large enough to display PDFs without doing much beyond trimming margins and adding a landscape mode. There are going to be many large screen EInk devices available by the spring of next year, and they are going to cost significantly less than $500. Note that the
enTourage eDGe has an 9.7" Wacom touch EInk screen and a 10" LCD screen and it will cost $490.
It is possible that 8" screens are also "good enough". The first real test of this will be the DR800SG, which I expect to get cheaper by the time it is introduced in Europe next year.