That is a very difficult question which often cannot be answered with real accuracy. Here is a good article provided by freeadvice.com on common factors used to determine the relative value of a medical malpractice case:

• Jurisdiction: Where the injury occurred can sometimes have as much to do with the amount of damages or settlement you can collect as the injury itself. The rules about unprofessional conduct and doctor liability vary from state to state. Your attorney will help you establish the venue and jurisdiction of your case and advise you on what, if any, limitations the state places on your potential medical malpractice settlement or damage award.
• Potential state caps: Since states are generally responsible for issues of medical malpractice, many have instituted caps on malpractice damages as a deterrent to frivolous claims. These include taking the amount available to an injured person through “collateral sources” such as health insurance out of the settlement, limiting the payment of damages to installment plans instead of lump-sum payments, and capping damages altogether. Check with a medical malpractice lawyer within the jurisdiction in which you were injured to figure out if there are caps on what you can claim in your medical malpractice case.
• Severity: Lately, insurance companies are increasingly attributing rising premiums to increases in claims of injury severity. Severity is the degree to which you were injured. For example, a cleanly broken bone that requires a cast is considered a less severe injury than the same bone that was shattered in several places and which required surgery and extensive rehabilitation. It’s a common-sense guideline for how much your case is worth – the more severe the injury, the greater your likely damages or settlement.
• Pre-existing conditions: As in workers’ compensation law and other areas of law, it will be harder to get a large settlement or award for an injury on top of a pre-existing injury. For example, if you had knee surgery years ago, it may be more difficult to get a large settlement for a case resulting in damage to an already problematic knee. Be honest about your pre-existing conditions, if any, when you consult with your medical malpractice lawyer – the opposing side will be asking for a detailed medical history including medical records and can capitalize on any pre-existing condition or injury related to your medical malpractice case.