The invasion of Libya was a costly enterprise for Italy. Instead of the 30 million lire a month judged sufficient at its beginning, it reached a cost of 80 million a month for a much longer period than was originally estimated. The war cost Italy 1.3 billion lire, nearly a billion more than Giovanni Giolitti estimated before the war. This ruined ten years of fiscal prudence. After the withdrawal of the Ottoman army the Italians could easily extend their occupation of the country, seizing East Tripolitania, Ghadames, the Djebel and Fezzan with Murzuk during 1913. The outbreak of the First World War with the necessity to bring back the troops to Italy, the proclamation of the Jihad by the Ottomans and the uprising of the Libyans in Tripolitania forced the Italians to abandon all occupied territory and to entrench themselves in Tripoli, Derna, and on the coast of Cyrenaica. The Italian control over much of the interior of Libya remained ineffective until the late 1920s, when forces under the Generals Pietro Badoglio and Rodolfo Graziani waged bloody pacification campaigns. Resistance petered out only after the execution of the rebel leader Omar Mukhtar on September 15, 1931. The result of the Italian colonisation for the Libyan population was that by the mid-1930s it had been cut in half due to emigration, famine, and war casualties. The Libyan population in 1950 was at the same level as in 1911, approximately 1.5 million.
Question: How many more lire per month did the invasion of Libya cost Italy than was originally estimated?
Answer: 50