This thread has highlighted a feature of UAVs that is not often discussed - their loss rates due to failure and enemy action.
Looking at UAV programmes over the years, there has been a remarkable reluctance by makers and users to admit to very high losses and the consequent costs. Global Hawk is a good example, Predator another.
Reasons? Some down to design - poor landing gear, difficult handling characteristics, unreliable electronics. The proponents of UAVs often argue that not having a pilot on board reduces cost and improves performance because you don't have to meet all those pesky safety and airworthiness requirements. Unfortunately, in some cases the resulting UAVs also don't meet our usual reliability (loss of vehicle) targets.
Once you get on operations, the military balance for these vulnerable devices is interesting. Flying a relatively slow and noisy aircraft over enemy areas means that they'll get shot down eventually. Bad guy fires off lots of AK-74 rounds (basically free) and we eventually lose a valuable ISTAR asset.
The U in UAVs is just that - 'Uninhabited' - it's not E for 'Expendable'. Things like Reaper are far too expensive to be that. It would be interesting to see the attrition assumptions being used in the MoD...