How much credit is necessary to give customer the cost price?

calc.py

import csv

with open('items.csv') as f:
    items = [{
        'sales': int(i['sales']),
        'cost': float(i['cost']),
        'price': float(i['price']),
    } for i in csv.DictReader(f, skipinitialspace=True)]

tot_sales = sum([i['sales'] for i in items])

rate = sum([(i['price'] / i['cost']) * i['sales'] for i in items]) / tot_sales

values = [30, 50, 100]

for v in values:
    print('Store {}, get {:.2f}.'.format(v, v * rate))

items.csv