Hmm, wonder if that's when production as a whole starts, or if it's just when she starts filming her scenes. And $300,000 sure doesn't seem like a lot, compared to what the men are probably getting, but that might just speak to the limited amount of screentime WW gets in the movie...
April/May is what's being mentioned as the start date for filming now. As to the salary, Warners typically pays better than Marvel/Disney. RDJ gets a huge payday because of how important he is to Marvel's success and the other Marvel actors negotiated pay increases after The Avengers
, but they first signed up to low-ball salaries. Chris Evans reportedly got $300,000 for starring in Captain America: The First Avenger