Will, I'm not naming names, but this issue was raised in connection with artists who gave up playing for a flat fee long long ago. The OP was referring to performers who operate exclusively on a percentage system - as is their right. Yes, there will be a minimum fee, to guard against those very very few rogue promoters who do no promoting (we call the result 'secret' gigs), but this will be way below the sum that both parties anticipate and upon which both the deal is being struck.

If the act has accepted a flat fee then, as I said above, of course it is the promoters business - though I don't think there's anything wrong with anyone - promoter, artist or audient - agitating politely for ticket prices to keep up with inflation and/or parity with events which offer a similar entertainment value.

Artists fee options are discussed here