that's a bug, firstly hold some money (eg $10000), then you upgrade your penguins and start a next stage, then pause the game and quit, you would go back to the main page and start the game again, you would find your upgrade is still here and you got back your $10000
 "Royal Offense"){kind=icon}
 "Caribbean Admiral"){kind=icon}
 "Dungelot"){kind=icon}
 "Tech&Magic"){kind=icon}
 "Pingy the Ping Idle RPG"){kind=icon}
 "Penguin Overlords"){kind=icon}