Unfortunately for people making a decision about a severance package, valuing potential legal claims at this early stage is more art than science. But here are some basic guidelines.
Generally, you should think about the total amount you could recover if you brought a lawsuit and won. An attorney can help you with this analysis. Sometimes this amount is less than people think, because your lost wages damages in a wrongful termination suit are offset by any other earnings and compensation.
Then you should consider the likelihood of succeeding on those claims- again, an experienced employment lawyer can give you a reasonable estimate on this.
The simple math is your claims might be worth the maximum recovery times the percentage chance of success. So if you have a 50% chance of recovering $60,000, you could think of the value of your claims as $30,000.
There are, of course, other considerations in making a decision about a severance package, including negotiating a good or neutral reference, certainty about the amount you will receive, and not having to go through the process of litigation.