It is a simple question with a complicated answer. Am I a market maker or a retail customer? What does my book look like if a MM? What does my clearing firm charge me to borrow funds. What do they give me on credit balances? How much margin does it require? What are my transaction costs? Do I have short option contracts to offset the debit balance. How long am I locking in 4% if I buy or sell it? I would only, as a MM, buy a .SPX box if the cost lines up as free money. There are more reasons to short the .SPX box.