arugula has some calcium but not a whole lot, around 32 mg in a cup. I think the difference is that arugula is eaten raw vs. collards are consumed cooked so you eat a larger quantity. A cup of cooked collards is around the equivalent of the calcium in milk. If you cook arugula down, you'll be consuming more and so, there will be a greater volume of calcium though I don't think it would rival collards. Sauteed arugula is delicious with a little olive oil and garlic. An entire bag of it cooks down to nothing so you can eat quite a lot of it cooked.