How much is your life worth? Is it worth laying out $80 and carrying 273 grams for example? This is the Outdoor Research Helium Emergency Bivy, a trimmed down version of their regular Helium Bivy (510 grams). If you carry a raincoat you could make a raincoat shelter for your head, and you could hold he bivy open with a willowy stick and a couple of lengths of foam insulation which has many other uses.
Most people who die on the trail are day walkers. The next biggest group are track walkers. Bushwackers being more experienced suffer much less. It is worth developing the experience, but also always being prepared for the worst.
A couple of other survival shelters to think about: This one at 350 grams and this one at 253 grams. Adventure Medical Kits have a number of cheap ultralight bivys as well which are well worth a look. For example their Escape Bivy (which breathes) and has some insulative ability as well – perhaps as much anyway as a summer weight sleeping bag (and Only US$60). They also have a lighter model (5.5 oz – 157 grams) for US$40. It is also a good way of adding some warmth to your sleeping bag and mat on colder nights. It is well worth having a detailed look at their site.
Mind you you can get a whole tent for 500 grams. One such as this for example if you are well-heeled, or one such as this if you are not. You would have to admit this little floorless mid weighing 410 grams which cost me US$43,50 or A$59.98 delivered is hard to beat
Here is another interesting DIY concept.
Mind you if you carry nothing else, carry one of these: If you could only carry two things in the bush, what would they be? AMC have their own model thermal poncho for US$8.