In the shopify admin you've to specify the "sell unit" price: 30$

This is the price Shopify takes to do the checkout, and we can't change it. But we can show another price in the product page: the "square feet" price

Sell unit is box of 10 square feet, so we have to show 2 prices:

a) Price of sell unit, which is 30 $

b) Price by square feet which is 3 $