How many overseas bases does the U.S. have and at what cost?

The Pentagon estimates that each year it spends $22.1 billion to maintain its bases and troops around the world. However, the actual costs even on conservative estimates is about $170 billion annually.

The totals could be even higher. Estimates of maintaining U.S. bases abroad since the start of the war on terror in 2001 range from $1.8 trillion to $2.1 trillion.
] The Pentagon must each year produce an "Overseas Cost Summary" that calculates the total price on overseas military activities. This includes costs of construction, repairs and maintenance. There are about one million U.S. military and Defence Department personnel and families abroad. This calculation for the 2012 fiscal year ending September 30 documented $22.1 billion in spending.
 But that is only part of the story. Congress conveniently ordered the Pentagon not to include the more than $118 billion spent in 2012 on wars in Afghanistan and other places throughout the globe. Even if you added this amount in, it does not come close to economist's Anita Danc's estimate of about $250 billion. Even without including war spending, Danc's estimate comes to about $ 140 billion, not even close to the Pentagon figures. 

David Vine who teaches at American University in Washington D.C. who has written for the New York Times, the Washington Post, and the Guardian, attempted to find out himself the cost of the more than 1,000 U.S. military bases outside of the United States and is currently completing a book on the subject.
 Vine talked to budget experts, and both current and former Pentagon officials, among many others in his attempt to gather relevant data. Many interviewed said that Vine's task was hopeless because of the huge number of bases involved, the secrecy of Pentagon budgets and the often fictional nature of Pentagon figures. The Department of Defence Vine notes is the only federal budget yet to pass a financial audit. Vine points out myriad missing parts of the puzzle in the Pentagon Overseas Cost Summary but I will just give a sample to keep this article of manageable length. I recommend the entire article at Al Jazeera.
Vine found that at least 18 countries and foreign territories on the Pentagon's own list of overseas basses somehow are not named in the OCS report. For example Kosovo and Bosnia. Another Pentagon report lists costs of $313.8 million there in 2012. That other Pentagon report also shows that the OCS report underestimated costs for bases in Honduras and Guanatanamo Bay by about a third. But there are even more omissions.
 The U.S. has a number of overseas territories that are often host to U.S military bases including Guam, American Samoa, Puerto Rico, etc. While these bases are domestic in that they are within the U.S. at the same time they are overseas. There are many bases on these territories, estimated to cost about $3 billion a year. A partial list of U.S. bases both in the U.S. and globally can be found here. The number of bases is difficult to determine exactly and depends on what is defined as a base but the total of of foreign bases likely is in excess of one thousand. The issue is discussed on some of the appended videos. The U.S. has by far the largest military expenditures of any country on the globe. Some of the details of the military budget can be found here.:

"The 2009 U.S. military budget accounts for approximately 40% of global arms spending. The 2012 budget is 6-7 times larger than the $106 billion of the military budget of China, and is more than the next twenty largest military spenders combined"