Question

There are more certifying bodies than I can count - and all of them are either worthless or worth very little in regard to producing quality trainers or trainers than can actually make a living.

If you are new to being a professional trainer, then yes - pay the $500 - assuming it is to a well recognized certifying organization (i.e. ACE, NASM, ACSM, ISSA, etc.)  If it is not a well know brand of cert, then go get one.  If you charge $50 per session, then the cert just cost you 10 sessions of work... that would be about one day to one week, depending on how much you are hustling - for 2 years of certification.  If you can't afford that cost of doing business, then you should look into another line of work.

If you are a somewhat experienced trainer (5 years of income as a professional trainer), then probably not - because you should already have a steady stream of clients, and a working relationship with one or more gym(s). But if its a big deal to a gym owner - do it... it should only be a day's worth of work... Plus it's just another business expense.

If this is for your own edification and improvement, then spend the money studying from the best trainers in your area (based on experience, expertise, ability to teach, etc.)  Learn from others who are actually good at training people, and don't limit it to "fitness."  Go to martial art studios, boxing gyms, gymnastic coaches, pilates, yoga, gyrotonic studios, aerialists, etc.....

Go to medical seminars, read medical books, and go to symposiums.

If you're serious, then get curious and learn from everything and everyone.

I've been doing this professionally for over 25 years, and am always learning or discovering something new.