In the context of the OP how much have we spent on Libya in the past 5 months?
A better question is, how much have we spent IN Libya in the past 5 months.
Much of the cost of these fights is spent either 1) in the US, or 2) on US people (soldiers, contractors, etc.). That money doesn't just evaporate, it is used to creat jobs in the US or in salaries to US people.
That doesn't justify the wars, and it isn't necessarily an efficient way to spend money, but I never see anyone who quotes these multi-trilion dollar costs telling us how much of that cost stays in the US economy.