How to sum all price values having in count the quantity of each one
You could always call on the pandas DataFrame's columns and work with that.

values = df['price'] * df['quantity']
sum(values)


if you want more information, I recommend https://stackoverflow.com/questions/14059094/i-want-to-multiply-two-columns-in-a-pandas-dataframe-and-add-the-result-into-a-n