Light, comfortable, cheap - pick any two. I am not so young anymore, and was dispointed by the comfort of the 1.5 inch Thermarest. After a few years, I bought a 2 inch, and it was a bit better. Still, if the ground is hard, it is tough on your back. So I am now hesitating between two new(ish) mattresses: the new 2009 version of the Exped downmat (in 7 cm regular size, so as not to make it too heavy). It is about 900 grams. Its biggest advantage is that it is filled with down, and thus had superb insulation quality (my current Thermarest is cold at sub zero temteratures). My hesitations concern the weight, and reported quality issues with the previous model of this mat. The alternative would be the new (May? 2009) and ultralight plus compact 6.3 cm Thermarest NeoAir, at just over 400 gram for a regular size. It has an R=2.5, so it insulates, but not as well as the Downmat.
If you want cheap because you don't really know what you want, and want to find out first, try to find a second hand Thermarest (they are good quality) - plenty of those around.
Willem