Flying to London is not expensive. After the recent change in CO award rules, I redeemed 50K for a one-way VS US-London flight in business class and I only paid $2.50. I wouldn't call that expensive .

Flying from London is a different story, as one has to pay UK airport taxes, the dreaded APD and various US immigration fees. In total, it would be close to $250 for a business class flight and about $100 less for an economy class flight. These numbers assume no fuel surcharges.