what was the cost of building the lusitania?

the British Government under Prime Minister Balfour authorizes a loan of £2.6 million for the construction of the Lusitania and Mauretania, providing that they meet Admiralty specifications. This comes under threat of German competition and monopolization of the North Atlantic by American financier J. P. Morgan, who has already bought out Cunard's main British rival, the White Star Line. The British Government also agrees to pay Cunard an annual subsidy of £150 000 for maintaining both ships in a state of war readiness, plus an additional £68 000 to carry the mail.

source and more info here

http://www.rmslusitania.info/pages/index.html

In 1903, Cunard chairman Lord Inverclyde took these threats to his advantage and lobbied the Balfour administration for a loan of £2.6 million for the construction of Lusitania and Mauretania, providing they met Admiralty specifications and Cunard remain a wholly British company.