However it's possible to imagine a situation when buying a straddle would make sense regardless of the subsequent implied vol contraction. At extreme, it would be sensible if you think there could be a move that will take your strike to low enough vega.
Anything is possible. But is it practical? I think not. If you believe otherwise, we agree to disagree.
Do you mind elaborating why do you feel that it is better to look at prices instead of implied vols?
The short answer is that in this particular case, if a binary event is going to drive premium down and possibly toward parity, it doesn't matter what the vols are. All that matters is that you're selling more premium than you buy, and if your wish comes true, you capture some or all of the spread
I can't give you a one size fits all answer because with each set up, time until expiration varies, and IV varies across time and price. So let's go with what elitenapper asked me:
"What would be the percentage in drop in options prices for an IV collapse from say 80 to 40, or from 50 to 30? Would that vary across strike prices or expiry dates?"
In order to give provide something more concrete than mere words, I crafted a hypothetical position where the IV across both weeks was identical (not gonna happen in the real world) and modeled some numbers. As posted above:
XYZ is $100, IV is 80 and the 1 week/2 week ATM short call calendar offers a theoretical $1.85 credit if IV is the same for both weeks ($6.28 - $4.43). The next day, FDA denial. Stock drops to $90 and IV drops to 40. Respective options drop to $0.06 and $0.31 for a debit cost of $0.25 to close the spread (ignoring B/A spread slippage to enter and exit).
Increase in the far week IV creates more TP, inflating the credit . Near week IV is going implode but dollar change is the focus not pct IV change. All that matters is the quantitative premium change in each.
Yes, the example is hypothetical. At this point, all I can suggest is that the next time there's a binary event looming, look at the numbers and if week two's expiration IV has also expanded severely, look at the possibility of the reverse calendar. The numbers may line up nicely, they may not. I did some of these in the days of trading single name events All worked in varying degree but yes, they were a finite sample done with pin money. It's not like there's an FDA announcement every week nor would I bet the farm on it.