I think there is a difference between originals bands & covers bands.
In my old punk covers band we used to contact new venues asking for a gig and tell them we charged £200 for the first one as an introductory offer, after that they could determine our worth if they wanted us back. They knew the music so knew what to expect, and must have known if we were suitable for their venue/customers. We were always asked back.
Whereas with originals bands, promoter asks a starting out unknown band “what do you bring to my event”. How do they justify a fee. “Well we’ve not played many gigs yet, don’t have a demo and this gig isn’t in our home town so we’re not bringing in many extra punters”. I can see why promoters wouldn’t bust a gut to get their wallets out. They could of course lie, but then the promoter could too…..