PSIA-E dues are $95/year. Once you are certified, the minimum number days of PSIA clinics will cost you roughly $100/season in fees. Add $100 minimum per season for clinic travel expenses. Figure $100 for the level 1 exam and $250 each for the 2 and 3 exams plus another $500 for travel expenses. Throw in another $600 (fees+expenses) to have enough prep clinics to actually have a shot at passing each exam. If you did it in 3 seasons, the minimum cost would be about $1500. Very few people pass with the minimum cost spent. Many people have spent 10 years and over $10,000.
If you work full time for 35 hours per week for a 16 week season, that's 560 hours/season. Out West, you could make up to $5 more per hour for a level 3 cert. My bet is that most 3 years of experience Level 3's would do well at a $2/hour difference. So maybe you could make it back in 2 seasons. But the odds are heavy against it.
My rule of thumb is to answer NO to anyone who asks this question because the intangibles are really what makes level III certification worthwhile. If you teach part time, you generally spend money to teach. If you're going to teach full time as a career and want to have some pride in the work you do, getting certified is ONE way you can grow yourself into the best instructor you can be. If you're going to teach full time and really need to do the math to see if certification is a worthwhile investment just because full time ski pros are usually dirt poor, then it's going to boil down to how much work you are going to put into the effort, what the pay scales are where you work, how much you can minimize expenses, how fast you can pass, how long you plan to teach, etc. It is most certainly possible for certification to make financial sense for career ski pros. Certification can open doors for you. Nonetheless, when the FIRST question is "Is it worth it (financially)?", that's the clue that the answer is going to be no. When you first ask the other questions about what happens to your teaching and the other things that certification lead to, THEN check finances, then there's a chance your worth it answer will be yes.
I teach part time. It's not likely that I'll teach full time until I can retire from my day job. I will never recoup my PSIA expenses in higher wages. But a level 3 cert would still be doubly worth it to me in the value of the improvement to my skiing and the value of the improvement to my teaching. I don't mind paying to see people smile as a result of what I teach them. My objective is to get more people to love the sport more. I intend to pay more in order to get better at this. And yes, I am completely and totally nuts. Skiing can do that to people. Crazy people don't ever have to ask "Is it worth it?".