For cost per unit, you divide the cost by the number of units. $20/(200 units) = $0.10 per unit. $60/750= $0.08, so 2 cents cheaper per unit.

But if you actually only need 50 but can’t resist a “good deal”:

The 200 cost $0.40 per item that you actually need, the 750 cost $1.20 per item that you actually need (plus the cost of storing and at some point throwing out the ones you don’t need).