How much is the buyer's premium, and why is the final price higher than the one in the room?

The buyer's premium is the auction house's commission, which the buyer pays on top of the hammer price, sometimes plus VAT. That is exactly why the figure that falls under the hammer is almost never the one on the invoice. Internationally it runs at roughly 27 to 30 per cent; in Czech auctions it reaches up to 30 per cent. When you read about records, the figure is usually already the final price, premium included.