I think many of you missed a key word in ralf's original question. He asked about bands that play only original music.
Naturally cover bands will make more than original bands on a local circuit (unless the original band has a large following or has some national exposure).
I've been in 2 original bands (both were several years ago) and we rarely made more than $200 per night for the band - and didn't work often enough.
Now I've been in lots of cover bands that made from $300 - $2000 per night.
Currently, I'm in 2 bands (and sub for several others). Our 3 piece jazz/funk group makes $300 per night playing 25% originals and 75% covers (many obscure covers). We book our own gigs, but we played for a few years to establish a rotation of clubs, before we introduced original material.
The cover band I'm in (5 piece - classic rot) makes a minimum of $1500 per night, but the booking agent gets 15%. It has always seemed the more interesting the group is musically, the less they earn. Whereas the crap cover bands make more than the music deserves (Although it still take musical talent to learn other peoples material).
So, I play originals and interesting covers on Friday night for less money, but more enjoyment and on Saturday night I play the same classic rot songs I've played for 30 years and earn more than I should.
But, isn't that why we all have home studios, to create original music.- - since most people in clubs want to hear the same old crap???