Your ability to negotiate is limited only by your creativity and your willingness to walk away.
A favorite tool to use by most employers is the "company policy." For most candidates that's the end of the road. It's either accept what's on the table or walk away.
There is still plenty of space in between if you take the time to get creative.
For example, a recent client was leaving a job where she had 20 days of PTO each year. The new employer only offered 15 days of PTO to new employees. She didn't want to lose those days because she needed to travel frequently to care for elderly parents.
"I'm sorry, it's company policy," was the answer when she first brought it up.
We needed to get creative. The company also offered up to 5 days for continuing education training and $2,500 for expenses. Our client traditionally completed her CE online and didn't travel very often to the national meetings. Her online CE courses cost about $250 per year.
She proposed using those additional 5 days to care for her parents, do her CE courses online while visiting them and the company could save the $2,250 difference it would have paid had she attended the national meeting.
A win for all involved. Creativity to the rescue.