The cheapest choice is the Kindle DX except there are some compromises. I paid $244 USD for a refurbished DX so that price is low. The DX is nearly obsolete and the software has not been updated by Amazon so that is a big disadvantage. The DX also only has 128M of memory and that could be a problem with browsing while I have never had a problem with just reading pdf files.
I have installed Duokan to read pdf files and that also works nicely.
The DX display is adequate except soon there could be large screen eink readers with 1600x1200 and that is what I would really like. The Hanvon E920
is advertised except not available as far as I know. That is similar to the Jetbook Color except without the color filters that cause a very dark grey background. The DX is just barely good enough for me until the E920 or similar reader is available maybe even with flexible eink.