Northern Dancer - stood for $10,000 (Canadian) in 1964 with a 'no live foal no fee' contingency.
20 years later his covering price had risen to $1m (US) with no live foal guarantee. This is by some distance the highest known fee ever charged for the services of a thoroughbred stallion.
(Taking into account the exchange rates of the time, and allowing for inflation, $1m back then is getting on for £1.9m now. Frankel's covering fee is currently £125,000.)