What Is the Theoretical Upper Limit for the Difference between the Futures Price and the Spot Price in Contango?

The theoretical upper limit for the difference between the futures price and the spot price in contango is the full cost of carry. If the futures price exceeds the spot price plus the full cost of carry (interest, storage, etc.), an arbitrage opportunity exists.

A cash-and-carry arbitrageur would buy the spot asset and sell the futures contract, locking in a risk-free profit until the prices are forced back into alignment.