I use crispy PU for tarps and groundsheets. It's excellent for tarps and pasasble for groundsheets.
If it's spinnaker then it will stretch a little if strongly encouraged. Nylon doesn't tend to stretch or tear.
Either way, it's likely to be 2oz or under per square yard.
If you are buying it to experiment with then consider that it might be worth paying $2-3 yard just to have a play.
It's not worth getting hooked on "is it or isn't it spinnaker". Instead consider is it light enough for what you want, is it strong enough and is it waterproof enough. Think about function.