I agree with all those that have said that the cost for professionally marketed rulers/shapes is in creating a precision edge. However, it is well known that using different brands of rulers on the same quilt can cause BIG problems because they may be off by as much as 1/8".
That said, in the "olden days", quiltmakers used to use cereal box templates, paper templates, or whatever they could get their hands on. It has been my experience that using thinner templates (without a ruler) usually results in the templates getting ruined pretty quickly. In addition, I found that trying to rotary cut with template plastic or something similar to that (even the thickest kind) tends to dull my rotary blade. What I do when I need a long line is I fold my fabric (remember that rotary blades can cut through multiple layers) and use my 18" ruler. For squaring blocks, I also make either a piece of tag board (for heavily appliqued blocks) or will iron together 2-3 layers of freezer paper & cut a template out of that, then iron that to my block. Because the freezer paper won't move, I can take my time moving the ruler bit by bit to get my block square. You can use dulled rotary blades to cut out the freezer paper to make that process faster.