For the lightest deadicated reader I have used the Kindle Keyboard and really like it for outdoor reading. I think the one I bought was around $99 a couple years ago.
I also have a Kindle Fire (last years model) and love it because it works not only with books but magazines and other media formats (like movies). It is a great with a low cost and most functions of a tablet and you can get one new for $199.
As far as light is concerned, I acutally do quite a bit of reading on the Kindle app on my Android phone. Since I am carring it with me anyway I would say the weight of the eReader function is 0.0 ounces. :) The down side is that it is rather small but you get easy "bite size" pages on the phone.
Across the board I like having Kindle compatibility since I do most purchases on Amazon and the Whispersync allows me to sync to the last page read on all devices.