From what I’ve heard music takes an incredible amount of space.
I rarely bother with it.
It doesn’t require money, it requires time.
The only reason triple-A games are so expensive are because of the amount of time spend making them and number of highly skilled people working on them.
Those people have to be paid because they’re doing it for a job.
If you’re making the game in your free time then all it costs is your free time (and a bit of electricity, and obviously the cost of the Arduboy itself).
You develop the skills yourself using free resources,
and you just try your best to make what you can.
An 8-bit version maybe.
Although it’s best to stay away from copyrighted music if possible.
I doubt Metallica would come after you with a lawsuit, but theoretically they could do.
It’s an 8-bit monochrome console, you don’t need to hire an orchestra.
All the tools you need to make games are free, and the forums and the internet are full of freely accessible information, and there’s loads of experienced people here to give advice and help out.
You don’t have to pay a single penny, but you have to be willing to put time and effort in to learn the skills, and/or be prepared to team up with someone who does have the skills.
Before you go overboard with designing your game, try to make a simple game first and get to grips with the basics.
It always seems easy when you’ve got the idea in your head, but there’s a lot of work involved and it’s often hard to keep motivated.
You can’t just jump in and make something amazing overnight, you have to develop the skills first.
Fortunately there’s people here who can help and offer advice,
so it will be easier than if you were trying to do it all on your own.