I'm not sure how it all works outside of the US, but the songwriter's share usually doesn't come from the "band payout". That belongs to members of the band after all the expenses are paid (costs above that John noted).
The songwriters get paid from performing rights agencies like ASCAP, BMI, SESAC and also from the publishing company. I don't take anything from the band when they get paid. My cowriter gets his share because he is the guitarist. He doesn't get extra for being a songwriter when the band gets paid for a performance from a club, bar, festival or wherever they are getting paid for their performance.
The performing rights agencies get their money to distribute to their publishers/writers from licensing fees they get from users of music (radio, cable and network TV, bars, clubs, shopping malls, concert halls, airlines, etc.). The value of the payout to the publishers/writers has several different factors, including how much they received in licensing fees from where your songs were performed.
These agencies will pay the songwriter directly for the performance, not the club that the band has performed at. As a rule, what the agencies collect, the publisher gets 50% and the writers split the difference of the other 50%. If you self-publish and you are the sole writer, you get 100% of the royalties.
This is why it "pays" to have multiple "jobs" in the music industry. Band manager, publisher, publicist, web designer, writer, performer, photographer, videographer, studio musicians...as many jobs as you can fill on your own especially when you are starting out. It keeps more money in your own pocket.
The agencies pay for performance royalties. Songwriters also collect mechanical royalties from the sales from downloads, CD sales and other sales containing the musical compositions. Mechanical royalties get paid to the record companies who pay the publishing companies (or representatives like the Harry Fox Agency) or if you self-publish, directly to you. The publisher shares these royalties with the writer (the amount being whatever is in the contract between the publisher and writers).
The statutory rate for mechanical royalties is 9.1 cents per song and so much more per minute over for a longer song.
Hope this makes sense and helps you understand a little bit from this end of the songwriting process. This is fresh in my head as this is the step I've been working on a LOT over the past several months. It pays to be be as knowledgeable as possible in every step in the song process regardless if you have a separate publisher or have been signed by a record company because you do not want to be ripped off.
P.S. Just because you register your songs with a performing rights society doesn't mean you can skip registering your songs with the copyright office...that's a whole other topic in itself...
Cheryl