Depending on how accurate you want to be/need to be, you'll probably want to buy some and weigh them yourself. There are many shapes and sizes of worms, with each having varied consistencies so the weights will vary as well, but very slightly. So, again, depends on how accurate you need to be.
My son just got a worm farm going. Real easy, fun for the kids, and a great source of bait and fertile soil. He's using red worms. Slightly smaller than nightcrawlers.