What is a mana token in Dungeon Defenders?

I've seen Dungeon Defenders players ask for mana tokens while trading. What is a mana token and how do you acquire them?

As far as I know mana tokens are items that sell for 200.000.000 mana or more in the tavern (anything over 600M will be lost due to a mana cap), they are used in trades of high value items, and generally come in 200, 400 and 600 million mana values. Anything over 600M mana cannot be sold to the tavernkeeper for full value, therefore 600M is the current limit on tokens that have an underlying redeemable value.

Mana tokens are not found as high value items but, rather, they are items upgraded through investing mana at the Forge. Investing mana does not add the full value to the item, and this is affected by the underlying base value of the item. The final token value will typically be around 2/3 of the mana invested. Weapons also appear to gain more value from invested mana, compared to armour. Thus, to make a mana token efficiently, you will wish to look for a weapon that has 150 (for a 200M token) to 200 (for a 400M token) upgrade slots and a reasonably high base value (consider items >3M). Then keep upgrading it until you reach the desired value.