Well, the cost of earning a Level III is not just the cost of the exams, travel, etc., but also the opportunity cost of the time to train to pass. Level III, at least in Rocky Mountain, requires a very high level of skiing and versatility, and you aren't going to achieve that level without a significant investment of time to train. Perhaps some could achieve that working full-time and training for no more than an hour a day (less when you consider lift time and the need to report in for work), but there's also the time that's required to study for the movement analysis and teach.
Many have highlighted the issues of older instructors achieving Level III, but there is one hurdle that hasn't been clearly highlighted: mindset. Let's face it; older folk have lots of experience, but that experience can also be a hindrance. Rather than embrace new learning, many of our older folk either can't understand the coaching (they get stuck on their own beliefs or simply can't reconcile what they are hearing with their own learned understanding) or are unwilling to try something new. Or, for that matter, try something new for long enough to move through the cognitive, associative, autonomous cycle.
Level III is not only about skiing at a certain level, it is also about versatility. Have the early pressure to the outside ski move down? Can you delay it? Mastered recentering by pulling the feet back? Can you do the same through flexion/extension moves of the ankle and knee? Got rotary down? Can you turn the ski with pressure and edging? Versatility generally requires moving outside your own belief system to explore and challenge your own understanding of biomechanics and physics. It requires exploring different movement patterns. Not things that are typically easy for someone who has been skiing, and possibly teaching, for decades.
That being said, many Level III (and 4/examiners) become stagnant and stuck in their own experience. The best coaches and instructors, in my opinion, are those that are always exploring, trying to enhance their understanding and skill. These are the folk who are true "professionals."
So, while all the other factors (pay, burnout, management, etc.) are present, experience can be both a enabler and a barrier to achieving Level III. It depends on your mindset.
Mike