The most important thing to remember is that a band gets paid based on their value to the people who are doing the hiring, plain and simple. That value may be monetary, or the coolness factor of the band.
In a club situation, does the band bring any business to the venue? Clubs aren't in the business of paying bands just because the band sounds good and has passionate, dedicated players. Bands are paid according to the revenue they help generate.
For weddings, corporate events, etc. where no direct revenue comes from the attendees, the band is paid based on what they add to the event. they may be fun to dance to, they may specialize in a certain style ('20s jazz perhaps) or their name alone has caché. Back in the day in L.A., it was always cool to hire Jack Mack & The Heart Attack for events, and they undoubtedly commanded top dollar based on demand for them.
Negotiating is a delicated dance, and sometimes best left to an experienced manager. Even though they will take 15-20%, their skills to get more money (or better accommodations) may be well worth the fee.
The Touring Musician negotiation suggestions are are undoubtedly useful in certain situations, but not at the club level. Unless the band can demonstrate that it will bring business into the venue, the booker won't be swayed by thoughtful, diplomatic talk. If the band attracts 50 people at a gig on a Tuesday, that could represent $500 in drink revenue, and the band is entitled to ask for a portion of that in return. Not all of it mind you, clubs aren't in business to break even!
I'm way ahead of you: bands aren't in business to break even, either! Well, let's consider a venue's fixed expenses: building rent, insurance, maintenance, product, licenses, electricity, advertising... not to mention employee payroll. Those are the basic costs of doing business, they're not optional, and they exist even when nobody walks in the door. Compare that to the only direct, out-of-pocket expense a band has per gig: the cost of gas to & from the venue. So if a band splits $30 in tips, they've covered the hard cost of gas.
Just trying to put in perspective that a band must seriously consider what they bring to the table, before assuming that venues are obligated to pay bands just to be fair.
Bermuda