Are all the bags the same size? 'Cause if so, those bags I saw in the little makeshift store by the ride are a bit small for that price. The one I did at War Eagle was $8.50, and it had to have been twice the size of the bags I saw at SDC.
Then again, it all depends on what stones are included in the bags. The one I did had sapphire and citrine and I think I managed to grab one or two emeralds; still not sure if any of the reddish stones are rubies, still need to do a bit of research to figure out.
So, depending on how much you can find in a bag, $8 may actually be worth it. Panning for gems really is a bit of a crapshoot, really. (is that an okay word to use? I'm really not sure... LOL)