Those blue and camo tarps aren't super heavy, but are definitely heavier than the tarps made out of silnylon, spinnaker and cuben fiber. If your on a tight budget they would work just fine as a lightweight shelter. Those expensive fabrics yield shelters from 4oz solo shelters to 12oz 8x10 palaces and are for when you can afford them, or are trying to shave every ounce. Shipping weight on a blue 7' 4" x 9' 6" tarp is 1.38lbs or 22.08oz from harbor freight. At 22.08oz it is definitely a lot lighter than any tent you'll pick up for $5 for one person. Which is probably why they have been in a few of the UL on the cheap posts. Another thing is that those lighter weight blue tarps like to pull out eyelets under a lot of stress. I use them for car camping all the time as shelters over picnic tables, as gear or bike protection, over cheap tents, or under the same tents in torrential downpours to keep the floor dry. Another downfall to the blue tarps is they don't always pack back down small or at least not as well as some of the other materials.
You don't have to spend tons of cash on a silnylon shelter either. Just pick up a cheap sewing machine and start learning to sew. It's fairly rewarding to make your own gear, and not to mention cheap in almost all instances.