When buying a stock (say Google or Apple) at day 1 you pay the ask price of that day. When selling the stock at day 6, you get the bid price of that day.
The return you made is given by [ BIDPRICE_day6 - ASKPRICE_day1 ] / BIDPRICE_day6
First, is this correct? Second, I am now wondering whether these principals also exist when going long or short in an equity index, such as the S&P 500?
If so, where can I find historical bid and ask prices of this index?