Definitely nothing wrong with the seconds silnylon. I used it for a poncho and have had no problems with it. It appears thru-hiker doesn't have it in stock right now, but OWF has it for $4.90 a yard.
And like Tim said, I don't think you car order partial yards from thru-hiker. I've never ordered from OWF, so I can't comment on them.(Edit: just noticed they only sell by the yard as well)
As for being worth it, I take a lot of pride in my homemade quilt and jackets (poncho-tarp not so much). The cost savings is nice if you have plenty of time, patience, and space. During the school year, I'm rather short on all three, so I don't make as much gear as I would like. Those are the times I don't mind paying a little more for something of higher quality than I could produce to receive it in much less time.
If you haven't sewn before, definitely have some one to teach you. I learned a lot watching my mom do the harder stuff while I did the shorter seams. And practice with the scraps a lot too.