If I will return to school, I will try to get into a good internship each summer. Many internships already pay higher hourly wages than the $20-25 I have been getting paid.
I am very self-taught myself, but as you can see it hasn't led me to a good career. Being the lone developer is actually a bad thing in the long run, and I learned the hard way that if you're the smartest person in the team, you should move up to a different team or a different company. I learn better by working with someone that can kick my ass at software development most of the time. I don't even know how to write unit tests!
But I'm also talking about the training done at big companies like Google. I expect their sort of infrastructure and resources to be top of the line.
It is a very long, and risky path to get there. Do you just want more challenging work? Do you want to do more math and less coding? You can more easily accomplish these goals.
And in terms of opportunity cost remember just cause you made $20-25 the last few years doesn't mean you will make that much over the next 4 years you would be going to school. And if you are willing to work hard you could be making six figures in two years by staying in web or moving to mobile.
Also your ideas of training in the software world are way off base. I don't know any devs who are any good from training.
For instance I did well getting a CS degree from a reputable state school, all my project and test scores were in the 90s.(UT Austin) But I didn't know how to program my way out of a paper bag. I learned this the hard way, failing spectacularly at my first internship. So I spent the next year working as hard as I could at learning how to develop. It was all self-training and it made a world of difference.
You'll find that all the best devs are voracious auto-didacts. 95% of all learning that happens in software is reading websites, books, and trial and error, not being sat down and trained how to do something. And the 5% who rely on training are usually pretty shitty devs, because you can't grow very quickly as a dev relying on other people to train you. The spoon fed training usually isn't that good, or complete. To be honest I don't know any devs who could even be generously called mediocre who relied on training/teaching to be good.