Just because an aircraft is "unmanned" doesn't mean it's "unattended". The cost of developing technology capable of operating large aircraft sans onboard pilot will be considerable, as will be the cost of maintaining the hardware. When you factor those costs into the equation, the expense of having human pilots onboard, even at $100-$200K/yr, is a heck of a bargain.
The reasons that UAV's are finding acceptance within the military has nothing to do with the cost of the pilots. If anything, it probably costs more to operate those vehicles, not less.