Square bales can weigh 60 to 130 pounds and round bales 1500 to 2000 pounds, so if you take an average (plus I am rubbish at maths) there are very approximately 18 square bales in a round bale.
Some clever Farmer HHo er may be able to give you a more accurate answer
I find rounds are more cost effective , alot of people say alot gets wasted but
If u store it propley such as pallets cover it up it doesn't get wasted
If u feed it whole to horses this is when it's less cost effective as most gets wasted trampled I'n etc