It's fun to place all blame on the USA, isn't it?
From the below link [1]:
Mine clearance teams in Afghanistan report finding literally dozens of types of landmines, mainly from the ex-USSR, but also from Belgium, Italy, US and the UK. The most infamous mine used during the Soviet Union's occupation period was the so-called 'butterfly' mine. Helicopter crews dropped untold numbers (figures range into the millions) of the small mines from the air.
Also, I found this interesting. Clearly, it's not enough to demine the world, but it's more than the zero dollar amount you basically assume:
[2] Despite its refusal to join the treaty, the U.S. has done more to counteract mines than any other country, spending $1.9 billion during the past 18 years through the Humanitarian Mine Action Program—roughly a quarter of the total spent on demining and other remediation activities around the world. There's been a special emphasis on helping Cambodia, which has received more than $80 million since 1993.
Reason for not signing treaty isn't too great, but there is hope:
[2] The American position is complicated. The United States has not used antipersonnel land mines since 1991, not exported them since 1992, and not produced them since 1997. But the nation has a stockpile of some 10 million land mines, and prior to the '90s, it exported 4.4 million antipersonnel land mines, an unknown number of which are still in the ground.
...
Nonetheless, under pressure from the United States Campaign to Ban Landmines, the Obama Administration has been conducting a comprehensive review of its land mine policy.
4.4 million exported, not clear if that includes how many they planted. This seems like it must be the majority, but it turns out there are an estimated 110 million planted in the world [3].
[1] http://www.afghan-network.net/Landmines/
[2] http://ngm.nationalgeographic.com/2012/01/landmines/jenkins-...
[3] http://www.unicef.org/sowc96pk/hidekill.htm