The back-of-the-envelope way to do this is to make a couple of simple assumptions. Define the "crash" -- say, 50%. Define the tenor -- say, over 6 months. Next, think about what vol would carry you there over that time. Probably around 75-80%, which I believe is in-line with where VIX topped out during Lehman.
Price a few different expirations -- 6 months, 9 months, a year, etc. Pick a deep OTM strike -- ideally one which maximizes your vega convexity and gives you the most bang for your buck. For a six month SPX option this is likely around 950 priced today with 80% vol.
Calculate the fair values for the options around 6 months from now, and compute your return %'s off the current market value of the options you selected.
These are lottery tickets. No one can predict a crash, and I wouldn't dump a lot of money into these. But owning a couple might not a bad idea. I would think you could easily find some options that'd give you 10,000%+ IF your crash scenario played out.
I'm sure others could give you a more technical, superior method; but this is my way of thinking from 50,000 feet.