(Please note that everything depends on the city you're in, the type and the size of the agency. These are all averages and ranges.)
Every creative I've met wants to be working on interesting, engaging and creative projects. You also have to keep your portfolio in mind. If you get paid a lot, but have nothing good to put in your book when you leave, then it can affect where you go next. Some hot agencies take advantage of this too. They know that they're hot and you're dying to work there, so they don't have to pay juniors as much. But in the end, it's probably worth it because you'll have great work for your portfolio and a name on your resume that gets people's attention.
When I was a recruiter, I would work very closely with a candidate throughout the process to make sure I knew what they were looking for monetarily. I would then get the offer approved, sometimes fighting for that particular number and if I then made the offer and was counter offered, I'd be annoyed.
Also, always, always be working on improving your portfolio. Ask people what they think and learn to take criticism well. You don't have to take the advice, but be able to hear it graciously.
Check out Ann's blog and call her up if you need any coaching.