This is a great question. I have read so many headshot photographers' blogs and I have to tell you: most of them lie. They don't charge such high rates because they have to or because they are that good. The reason is simple: demand.
Remember: no headshot photographer, no matter how good, can charge more than a couple hundred dollars for a session when they are first starting out because they don't have a large enough client base. They slowly grow their base by providing consistent quality and good word of mouth, and then they raise their rates to match the demand.
I'll be honest with you because you have to understand: no photographer is going to charge less if they could get more, believe me. Skill has nothing to do with rates, time does. The photographers who charge $750 for a session have been shooting for years!
Now, granted, during those years they have developed a skill. But at the same time, many of them might not have a gift and might not be providing as good of quality as people who charge less but have only been shooting for one year.
When you are looking to hire a photographer, most people hire based on price. They assume that cheap means bad quality and expensive means good quality. That isn't always the case. you should look at how long the photographer has been in business:
If a photographer has been in business a long time and has high rates, that's a good sign. If they've been in business a long time and have low rates, that's a bad sign.
If they've been in business a short time, just look at the quality of work and don't worry too much about the rates being high or low. Just go off of what you can afford.