I am a PhD student, so not much business background, and I am pretty strapped on cash. But, I asked myself, how upset would I be if I didn't spend a few hundred dollars and that was the difference between getting and offer or not at my dream job?
I did two sessions with a case coach after my first round at BCG, and before the final round, and received an offer. I would say it was definately worth it, since I think it was the small, advanced tweaks that helped put me over the edge to get an offer.
I think booking a case coach before you practice on your own to the point of being very advanced would be a waste of money, since you can learn the basics for free. I think the value of a case coach is helping you go from good/great to exceptional. There are certain aspects of casing that our peers just can't help us with because they lack the experience, expertise, etc.