Why is Spy cheaper than SPX?

Can you elaborate on why the price of Spy, the S&P 500 ETF, is typically lower than the SPX index itself? Is it due to the ETF's trading costs, the fees associated with managing the fund, or a combination of both? Additionally, does the difference in price between Spy and SPX vary over time, and if so, what factors might influence this variation? Understanding the dynamics behind this pricing discrepancy could help investors make more informed decisions when it comes to investing in the S&P 500.