From Hack and Cast Wiki
Here's where you can exchange:
- 100,000 Gold and 20 Gems for 1 Travel Point.
- 2 Platinum and 200,000 Gems for 1 Bid Token
- 500 million Gold for 1 Platinum Bar
- 1 Platinum Bar for 500 million Gold
You can't exchange Platinum Bar to Gold if you have over 2 billion gold on you.
You can also see how many of them you can afford.