Interestingly I could not locate quantitative data on Firing Costs (aka Separation Costs aka Severance Pay) for the US Economy. Perhaps because, as US Dpt of Labor says
There is no requirement in the Fair Labor Standards Act (FLSA) for
severance pay. Severance pay is a matter of agreement between an employer and an employee (or the employee's representative).
A world map and another one for separation costs measured in weeks of wages -but none of the two include data on US Economy.
ILO's new data base does not seem to have quantitative data on Firing Costs.
Kugler, A. D., & Saint Paul, G. (2000). Hiring and firing costs, adverse selection and long-term unemployment. deal with some US data but qualitatively and indirectly.
Abowd, J. M., & Kramarz, F. (2003). The costs of hiring and separations. Labour Economics, 10(5), 499-530. Data from the economy of France, related to year 1992. Extensive treatment of various costs associated with labor, including costs of separation treated distinctly. Note: monetary values are in French Francs. Conversion Rate: 1 FFr = 0.152449 EUR.