The Rich Girl — R.L. Stine

Fear Street -- Where Your Worst Nightmares Live...
Emma and her best friend Sydney always share their secrets. And now they have a big one: They found a duffel bag filled with cash and swore never to tell anyone. But Sydney broke her promise -- she told her boyfriend, Jason.
Now Emma is terrified. She doesn't trust Jason. She knows he would do anything to get the money for himself.
Even if it means killing someone who gets in his way...