According to Adam Smith's Wealth of Nations, the war cost £40 million. The book was first published in 1776, 13 years after the French and Indian War.
"The great property which they possess both in French and English funds, about forty millions, it is said in the latter (in which, I suspect, however, there is a considerable exaggeration ), the great sums which they lend to private people, in countries where the rate of interest is higher than in their own, are circumstances which no doubt demonstrate the redundancy of their stock, or that it has increased beyond what they can employ with tolerable profit in the proper business of their own country; but they do not demonstrate that that business has decreased."
This is rough, and I could give you a better number if I looked for the CPI of the British pound.
$40 million in 1774 = $1,060,572,890.03 (2008)
$1,060,572,890.03 ≈ £718.4 million
Of course we spend that much every three days in Iraq. What's a few billion.