Quote from z32000:
if I don't sell and instead let the computer automatically close my contract, I will be getting the 805 back since that's the value of SPX at that time (multiplied by 50 since it's the ES)?
Yes, with some minor caveats. Read the link I posted earlier for more details. You bought at 800, you sold at 805, you would make 5x$50. Keep in mind, you did not actually pay 800x$50, so you will not be receiving 805x$50. Instead, you simply posted a performance bond and will make the difference.
In reality, however, you will almost always pay more than cash for a futures contract because you have to pay for the cost of carry. How much more you pay in carry depends on how far away from expiration you are.
Think of it this way: Instead of buying 500 stocks and holding them, you're buying a futures contract representing those 500 stocks. You and a guy holding those 500 stocks will both make the same P&L, but there's no way your positions are equivalent. The stock guy is putting $70k into stock while you are happily sitting on your $70k and earning interest on it.
Arbitrageurs figure a premium based upon the risk free interest rate for futures above cash for this reason. As a result, cash is almost always lower than futures, linearly converging to zero difference as expiration approaches.
Commodity markets also have carry charges resulting from insurance, storage, and risk free interest.