How much is being payed to the private security firms currently in Iraq and Afghanistan?

Is this money shown in a budget? Does it come from the DOD or from some other source? Why are we paying these firms to provide security and at the same time paying the military (much less) to do what should be a military function. With the largest mlitary budget in the world, why is it necessary to pay another force to provide this service? Why isn't this being considered for cuts in budget discussions?