When you say:
"the costs of putting on the show should be included in the ticket price and that includes the cost of selling the ticket."
how is a promoter supposed to account for the fact that if the tickets are available through multiple outlets, the price will be different?
I'd quickly run out of space on my flier if I said "£5 in advance/£7 on the door, but £5.50 in advance if bought from wegottickets, £8 in advance if bought through ticketmaster, £7 in advance if bought through folkgigsuk, £15 in advance if bought from some dodgy tout outside the tube down the road from the venue" etc etc

How about trying:

The price of a ticket is £10, no matter how you buy it.
1, If you buy from me early, I get the full £10, you know you've got a place, and you know I've got the full payment.
2, If you buy through a ticket agent, then that reduces the payment I get, because I discount my charges to them to prevent them adding a premium. I do this because they do increase the sales of tickets, and increase the chance of giving the performer a full audience. (I'd rather not, but that's the way the cookie crumbles)
3, If you buy at the door, it's still £10- but you risk getting turned away, because if I relied on 'on the door' sales I'd have no idea of the expected numbers.

Okay, I don't put on gigs, but the above seems reasonable to me (as a consumer). I'd rather know the money was going to the club putting on the show, rather than benefitting some middle-man (although that may be a necessary 'evil')