Calculate average per unit cost

I want to calculate the true average per unit cost for future pricing. 

We have an Inventory table, we buy "item A" at different price at different time

item A : 1 @$10 = 10 total 

item A : 1 @$20 = 20 total 

item A : 2 @$40 = 80 total 

The "average per unit cost" is different depending on number of item sold. The minimum price we can sale (with no profit no loss)

if nothing sold, average per unit cost = (1x$10 + 1x$20 + 2x$40) / 4 = $110/4 = $27.5 

if sold 1 unit, (1x$20 + 2x$40) / 3 = $90 / 3 = $30 average per unit cost

if sold 2 unit, (2x$40) / 2  = $80 / 2 = $40 average per unit cost

if sold 3 unit, (1x$40) / 1 = $40 average per unit cost

How to I calculate average unit cost in quickbase? Any way to do this with build-in quickbase function alone?

Much thanks!