Oh, when it comes to Arrow, I don't think its got to do with money at all. With Supernatural, yes, it did. But they (Arrow) signed her on as the female lead; they knew she was a bit more exspensive than some of the other actresses that frequent CW shows because of her wider range in film and TV. When they renegotiated contracts they could have changed her status (since supposedly EBR is now leading lady she should technically be billed second), since by that time EBR's 'importance' to the show had heightened. So if it was financial, they would've dealt with it during contract re-negotiations and adjusted billing accordingly.
No, if they are indeed killing off Laurel, its because of the following:
1. They royally screwed up but rather than be adults and admit this, they want to kill her off and maybe bring in a different version from Earth-2 who will go on to be a hero, erasing four years' worth of character development.
2. Guggenheim and his crew of hack writers seem to have the opinion the loudest voice are the most populous (which if you take a look at anything, whether it be entertainment, politics, or religion, is NEVER the case) so they do whatever the dwellers of Twitter and Tumblr want.