I think, for the majority of streamers, the way to set expenses is by looking at long term subscribers rather than day-to-day donations - those donos can fund fun things but you're going to want to try and keep your life expenses carried by the regular subscription income. A lot of the people who have gotten successful doing this have endured extremely lean times when they were trying to break into a decent sized audience - every streamer I've ever heard talk about the financial side of things plans things extremely conservatively.
Aren't subscriptions basically just a monthly donation? What do I get for subscribing apart from the icon in front of my name in chat and possibly a shoutout?
Absolutely nothing! But subscribers tend to be more reliable about re-subscribing.
When it comes to twitch you're never really buying anything concrete with your cash. You're mostly buying attention or a reply - sometimes you'll buy game effects or challenges, but usually you're just abstractly throwing money at them to keep the content coming.
