Since many people reported of having problems unlocking the secret character Monster Savings the developer gave an official statement on how to unlock that pilot.
The official conditions reported by the developers for unlocking Monster Savings:
Collect at least 500 coins with Bargain Hunter in game. You don’t have to collect the coins in one run.
We tried to unlock the character with that conditions, but we were not successful on iOS. Most probably there is a bug in the current version. The developers are informed and are working on it. We will inform you as soon as we have any new information.
If you have problems unlocking Monster Savings as well, or if it worked for you with the reported conditions feel free to leave a comment!
UPDATE: The unlock finally worked on Android. We collected 500 coins, we did neither watched any ads, nor opened any gifts for coins. We did also not spend any coins until Monster Savings was unlocked.