What is the monthly accident insurance cost for a healthy man that is 31?

Accident insurance is typically very cheap because it restricts coverage to a claim involving death resulting from an accident which has a low statistical probability. Whether or not a man is "healthy" is of no relevance for "accident insurance".

There are actuarial tables available that show the chance of death by accident by age and gender.

If you are asking about "accident insurance" that will replace your income if you are hurt in an accident, that is typically very cheap as well.

I am certain that you can get instant online insurance quotes for "accident insurance" if you use a search engine to seach the phrase "buy accident insurance"