Unless I'm missing something, the plaintiffs need to brush up on their math.

As far as I can tell, their claim that these fees "could increase the cost of a purchase by as much as 150%" is based on an example (#97 in the complaint). It says that "Ms. Smith once paid $31.50 for a ticket that the Red Sox initially advertised as just $21."

Yes, $31.50 is 150% of $21. However, this represents an increase of 50%, not 150%. So it would have been accurate to say that in this example the final cost of the ticket was 150% of the advertised cost, an increase of 50%.