It costs $2 from my lfs for one scoop. They say its about 2oz (or 1/8 lbs), but I never checked the weight. That amount can last up to a month, maybe more. no need to overfeed.
blackworms are freshwater worms, hence are not natural saltwater fish food. when I first dropped them in my tank, all the fishes got freaked out, its like I have a coral with live rock only tank. no fish in sight, including the copperband. they eventually got used to it and started eating. I only change the rodi once a week.
another option is to get the freeze dried black worms. copperband eats them just fine. live blackworms die within 10 seconds of hitting saltwater, so after 10 seconds, its probably not too different from the freeze dried worm.