2 Green and brown tarps for 10$ and a Bug net for 8$. 18$ and your set. Its no 200$ tent but its better then nothing at all. 70$ is just a bit much for me to consider "cheap".
Absolutely right, but are you seriously going to endure the weight of those tarps in your day pack on the off chance that you might need them? In so many items, there is a relationship between weight, cost, and utility. You can choose any two characteristics.
My personal preference is a SOL Thermal Bivy, usually listed for about $35, weighing about 6 ounces. It can be used virtually anywhere and will do better than a tent in retaining heat. I usually have a mylar blanket along which can be rigged as a tarp to keep additional rain off.
I also keep some blue tarps in my vehicle for possible shelter use. Weight is not an issue there.
You can find an assortment of bivy sacks, some of which are actually mini-tents, ranging in price up toward $200. Something for everyone.....
"Tents" are available very cheaply, often well under a hundred bucks. The ones I have seen are pretty much junk, useless and undependable once you leave your backyard, and heavy to boot.