I would replace "is" with "could be." Many, many people don't have a 30% marginal tax rate. I don't even come close.
.....but the ones able to afford the $1400 extra are likely to be high income or high wealth people, which is correlated with being in a high tax bracket. Of course, some people are in even higher tax brackets. My comment wasn't intended to be a precise statement, but just a reminder to consider taxes in such calculations.