They really need a family plan. A $100/month family plan would be easy to justify as "it's for the kids". It'd be more effective if parents did it with their kids and it would let the parents use it without having to commit to "getting their money's worth". Once you get over $200/month, that starts to be more expensive than other kid's activities.

if you're an engineer, The $50 is a pittance. your time is the much bigger investment.

But lets put it in concrete terms. lets say you want to break into machine learning and deep learning but you lack the requisite math background.

if you start at foundations 1, if you put in around 8-9 hours a week of work into it, you can get to the end of math for machine learning after a year and a half. lets be cynical and say 2. At 50 usd x 24 months, you're looking at a cost of $1200 usd. Even the cheapest community college set of courses you'll be taking to get to that level will cost you more than that AND will require a ongoing time commitment where you show up at the classes. While its nice to have a class, Mathacademy is much more efficient for getting the information into your brain. You have the problem set in front of you and you see imediatly if you get it right or not. the time window for correcting yourself from feedback is tighter than you're ever going to get from a classroom where your graded problem sets come back days later.

AS for return on investment, Just look up how much machine learning engineers make. coding is the easy part. Having the mathematical maturity to understand why certain things are used where they are and kwowing enough to deviate where they don't work is PRICELESS.