If they advertised the salary range, I probably wouldn't bother unless the job is one that will be tough to fill and you have skills that are hard to find. It also depends on how much $15k is in relation to the salary offered. If they're offering 20% less that what you need, you probably aren't going to be able to negotiate enough to get what you need. On the other hand, if it's within 10-15%, you might have a shot.