The CFO at the company I work for tried to jerk me around by saying they wanted to put off bringing me on permanent, a $400 a week difference, for months. The company's BATNA in this situation was even worse than the status quo. It took them four months to find me and I've been doing great. I made some token efforts at negotiating, then started playing hardball by calling the staffing agency and threatening to leave. They folded practically instantly.