In our experience, no one can really estimate the future cost of medical care. A lot depends on how you respond to treatment. If you are asking about settlement value, I would say that with some small cases you can estimate your own injuries and settle the case without an attorney.
What a board-certified specialist will be better able to do for you is to evaluate the value of the case in the particular market (i.e. where the case will go to trial). He or she knows this both by their own experience and by subscribing to numerous publications which report on settlements and verdicts. With this type of information, you will be better armed to negotiate with the insurance company in larger, more serious cases.