yeah, shipping costs have kept me from buying many times. But, eventually the itch gets the best of me...
To the OP, I have bought wally world fabric or Joan's fabric for projects. I lucked out with some good stuff from Wally world that I have used successfully. I even got some sylnylon. However, I have found that the sylnylon was not as waterproof as I needed for a tarp. So, for my last tarp, I bought the good stuff.
Like OM said, when you put as much time and effort into a project, you want it to last. That will only happen with proven materials. However, if you are DIYing anything, you will go through iterations. Now I use the cheap stuff to do mockups. So, go ahead and buy the cheap stuff and make something. And, know that you will change the design, and eventually you will want to buy the good stuff to make the "Best Pack Ever!". Make sure and take pictures so we can see your success!
As for what materials do I use in a pack... I actually use many different types. Why limit yourself to one type? A pack has different areas of stress. Some areas need to be more abrasion resistant than others. Some areas take more pulling. I like to use a heavier cordura, and a lighter ripstop, along with nylon webbing. I have different packs for different types of backpacking. Sometimes I use 1000D cordura for best abrasion and stretching resistance. Sometimes I use the lighter 500D cordura. Sometimes I am looking for a specific color and can only find it in 1000D. I only use the heavy stuff for the sides, bottom and sometimes the hip belt. Everything else gets the light mesh or ripstop nylon to lighten the pack.
For a light pack, you can successfully use just ripstop nylon. 1.9 oz would be more heavy duty than 1.1 oz. If price is a concern, use the 1.9 exclusively. It will be cheaper.