If they are scraped they are not worth much. Usually the case for buying/selling big text list of emails. Anyone can fire up scrapebox & get 1,000,000 emails at the end of the day. Not easy to mail & hit inbox.
If they are opt-in then they are worth something. But people who have that generally don't sell the list emails outright. They sell you an email --- you write an email & they send it to their list. Some people call this "email drops". Marketplaces exist where you can see what others are paying and go from there to determinate your rate.