I've been dropping the equivalent of a car payment for three months and it's been worth every single penny; even if I never actually sell a product.
https://github.com/ByteTerrace/Puck
It was originally a small project in scope, just a basic GameBoy emulator, a shader VM, and some Azure integrations. Just a bunch of things I wanted to dabble with in order to learn and have fun. Over the past few months it has grown immensely into something resembling an operating system more than a toy engine. There's even a custom DSL for authoring worlds that abstracts over the state/rendering models; it's completely ridiculous!
Will I make any money off the thing? Probably not, don't care. The things I've learned along the way have been worth way more than the amount of money I burned on a few monthly subscriptions. Learning how to interact with agents and treat them like a team was worth it alone; the product I have now is simply a nice bonus.