Consideration isn't a synonym for gain or profit. It requires an exchange, a trade. The commission paid isn't paid for the promise, it is incidental to it. It may have happened because of the promise, but not in consideration of it.
"If you buy a car from us and are unsatisfied for any reason, we'll give you back your money." You buy the car, relying on the promise. They refuse to give you back your money for the car. Consideration, or no?
Nefario said "if you hold the fake stock, we'll take it back in exchange for real stock". Goat bought the stock relying on the promise.
Nefario was specifically intending to increase the value of the fake stock. His concern was that people had paid more than it was worth and chose to handle it by increasing the value. Of course, he should have frozen trading prior to doing this. This mistake justifies voiding the promise, and that's why Goat is not entitled to lost profits.