It all depends on what work there is at the time and who it's for.
I have set rates that I charge for Rehearsals, Gigs, Tours, and Recording Sessions. But it's all fluid. Some people will negotiate lower rates if they can't afford you. And others will pay you more.
I don't ever give people my rates unless they ask outright for it. I typically ask what their budget is and negotiate from there. I don't want to scare people away by pricing myself out, but I also don't want to sell myself short if they have the money.
How much you make in a month will never be set. If an artist you work for decides not to book a gig, you are unemployed.
The number changes radically based on your market and location, your band's genre (and if it does originals or covers), and the venue. Numbers from forum members are almost meaningless due to these multiple factors.
On a good month, I don't have to use my paycheck to buy food or beer. On a bad month, I do.
I just asked a user on drum subreddit and he got 400 for one larger sized gig for a big convention like room. I feel like so long as your just playing some music as a side attraction or background noise it's going to be anywhere from 100-500 a night, depending on size of venue/local business or large corporation.
But as others mention, the worst part is that it's more or less unpredictable so you can't factor it into your budget unless you can get a consistent, narrow window. And even then you will be bummed if you're consistently taking in 500 a month and one venue gets shut down or something and all of a sudden its only 200-300 a month...
It varies and many weekend warriors with other jobs I guess take what they can get.
The Musician's Union in Norway's standard recommended rate $550+ cost, rehearsals etc... No problem charging that for one offs, but on the bar scene anything goes I guess. Arguing about the paycheck after the gig happened way to often when I was doing that stuff.
I have a consistent weekly gig as a house drummer for a place, so I am garunteed at least $500 a month with that. I'm in a couple of bands though so I can usually make like $700 or $800 a month depending on how many gigs we can get. The most I ever made in a month was like $1100 or so I think? That's because a school hired me to do a performance, my bands landed some gigs and I got the opportunity to do some recording work. Plus the constant gig I talked about earlier. I'm making a lot and I'm really happy about it, but I am always trying to make more. I have a friend who's the leader and guitar player of a local jazz band, and I've seen him make $800 in tips alone in just one gig, which is insane. I always try to make at least 100 per gig, but they key is to try and be involved in as much as possible I think.
for me, it depends on the gig, ive made as little as $50.00USD per show and I also have made as much as $700.00USD for a 2 hour set, for about two years I supported my family by just gigging, the absolute most best way to make a living!