How much did it cost to fund the hundred years war?

During a part of that period, England could fund the cost of war by its raids on French territories, where everything of value that they encountered was carried off to England. Nobody kept exact count, but the loot was big enough not only to initially pay for the war, but also to make many people very rich. Only later on, extra taxes were needed in England to pay for the war.
The cost in real terms is almost impossible to compute; think only of the 'loss' to both countries' economies because the soldiers - often recruited from the ranks of farmers and artisans - could not be productive in their normal jobs. It IS known that both countries ended deep in the red. The English kings - despite earlier profitable raids - had to borrow in today's money billions of pounds from bankers for the upkeep of the army and finally defaulted on those debts. France was totally ravished and paying the ransom of the French King - taken captive at the Battle of Poitiers - alone cost France more than its total revenue over several years.