I must disagree. If my reading is correct, it is outrageous--not the 0.5% increase but the rate structure. It looks like HA has hidden a huge total take by dividing its fee between buyers and sellers.
By way of comparison, this is what Christie's charges--with no seller's consignment fee (or at least none which I can find). It is from https://www.christies.com/buying-services/buying-guide/financial-information/:
"For all collecting categories except wine, the new buyer's premium rate will be an amount equal to 25% of the hammer price of each lot up to and including £175,000/US$250,000; plus 20% of the hammer price from £175,001/US$250,001 up to and including £3,000,000/US$4,000,000 and 12.5% above £3,000,001/US$4,000,001."
It seems to me that if the HA is taking 10%, plus 20%, that's a lot more than the maximum of 25% on the total by Christies.