I think options behave like futures only.
if you buy futures next month expiry is dearer than this month expiry.
Similarly next month ITM calls would be more expensive than this month
If you sell futures next month expiry gives more premium to us and hence is more beneficial. Similarly next month itm puts should be cheaper than this month ITm puts
provided we assume next month has more premium than this month.
I think costliest should be deep In The Money with the NEAREST expiry because it has only intrinsic value left with no premium. So the cheapest option becomes the costliest option in the other leg.