Holts do seem to have a great business model... 15% or so for the seller, 23% for the buyer, plus extras for photos and catalogues. Let's say 40% of the hammer price on average, and they have 4 big sales a year.
What I can't understand is some of the hammer prices. Modern shotguns for example, seem to go for close to retail price, which considering the commission just doesn't make sense!