To put a value on a case, an attorney will draw upon his or her knowledge about the type of case, the particular employer, and the jurisdiction (the state and court in which the case will be brought).
The amount of recovery varies with the type of case. For instance, a disability discrimination case under the Americans with Disabilities Act may allow for punitive damages (damages to punish the employer), while an age discrimination case under the Age Discrimination in Employment Act will not allow punitive damages to be recovered. When you remove the possibility of recovering punitive damages, the value of the case is reduced.
The amount of recovery varies according to the employer, as some employers refuse to settle and tend to be more litigious - willing to fight it out in court - than others. Additionally, if the employer is bankrupt or very small, their ability to pay may be very limited.
The amount of recovery varies according to the jurisdiction (state and court) in which the case arises and will have to be brought. For instance, in New York, your potential legal claims are more limited than in a jurisdiction such as New Jersey. New York is very employer friendly and does not allow many wrongful discharge claims, while New Jersey is known to be more employee friendly.