Fenrir, You want small deflated volume, high inflated comfort, and low cost.. there is really only one choice, inflatables. So I agree with the previous posters. Montbell's version has 6 individual chambers, so if one bursts you can sleep through the night and fix it later.. I own one and it is very comfortable. If you are sleeping in an tent, how could it ever get punctured?
Pressurize it so you just barely do not touch the ground, that is maximum cushiness.(sorry to get technical)
470 gram Inflatable:
Light weight is not so important on a bike, but the lightest 180cm combo in the world is the Montbell 120cm pad, 20cm extension, and 30cm pillow..
Choose the components from that same page link...