What is a rough cost on a cement slab 30' X 56'?

It would vary with the cost of concrete in your area and how thick the slab would be. I assume by the size that you are building a shop or similar. If so, I would go at least 6" thick.

1 cubic yard at 6" thick covers 6 square yards. So, you will get 6 square yards of concrete out of 1 cubic yard.

You have 1680 square feet, divide that by 9 for square yards = roughly 186 square yards.

186 square yards divided by 6 = 31 cubic yards

At $110 per yard here in the midwest, it would cost $3410. But, I would figure extra to make sure you had enough when pouring it. Possibly 1 or 1 1/2 yards extra.