'Thirty' in the Bible

Someone asked me, "What do you see?" I replied, "I see a flying scroll thirty feet long and fifteen feet wide."

And I said unto them, If ye think good, give me my price; and if not, forbear. So they weighed for my price thirty pieces of silver.

And the LORD said unto me, Cast it unto the potter: a goodly price that I was prised at of them. And I took the thirty pieces of silver, and cast them to the potter in the house of the LORD.