Say that a SPY butterfly has a midpoint of 0.94. Would it be reasonable to ask for a 9.4 fill on the same butterfly on SPX at the same strikes (x10)? Is this a good way to determine what might be the fair price on an SPX spread?
Beyond the obvious different in contract sizes between SPY and SPX, there are other differences between SPY and SPX that make direct pricing comparisons imperfect. For instance, SPY is an American-style option, while SPX is a European option. SPY pays dividends, while SPX does NOT. Furthermore, unlike SPY, options on SPX stop trading on Thursday and the settlement price is based on Friday's morning SET price. - Backtest options trading strategies
All of the above can be trivially accounted for, every MM out there hedges SPYs options with SPX options and vice versa without any problems. In most cases, with simple ajustments/approximations the discrepancies above will not have a material impact on the trade performance at retail level. In case of a fly, after adjusting the strikes for dividends (get ETF % strike relative to forward on expiration, impute strikes for index), you should be pretty close - the AM/PM expiration is not going to effect it too much. For a naked option you would have to assume some moneyness to decide on the early excersize, at which point you might as well just calculate implied vols and re-price index options instead. My 0.25 vega.