The ransomeware makers can counter that by making their ransomware divide the victim’s data into a very large number of separately encrypted sections each using a different key. Then instead of trying to sell the victim one key for a million dollars they would offer say 100000 keys at $10 each.

Now it only costs the victim $10 to pick a random section and buy its key. If that works they can pick another and buy it’s key, and so one until they are confident the seller can be trusted and they can then buy the remaining keys.