There's a "buy me a coffee" on the page if you're feeling generous!
It's true the cost is certainly the dev time not so much the servers. Though open sourcing doesn't protect things from dying - it's people not using them!
Open sourcing is more to enable people to be able to build the thing in the future with new libraries. OSes move forward every day and something that runs/builds today might not be able to run/build tomorrow, so open sourcing makes sure it's possible to change libraries/API usage if needed.