stock_data = '....................100 .......513.25 ..........'
# Вариант 1
cost = int(stock_data[20:32]) * float(stock_data[40:48])
# Вариант 2
SHARES = slice(20, 32)
PRICE = slice(40, 48)
cost = int(stock_data[SHARES]) * float(stock_data[PRICE])