NJHr, ignoring the proper aircraft jab, UAVs are multi mission platforms doing far more than simple EO/IR ISTAR. The more you do the more you save as the ever growing crew can all operate from home base, thus avoiding the additional manning burden and training/ops costs. Of course this has to be balanced against satellite bandwidth costs, but given that you are already sending full colour FMV anything additional tends to be minor. One of the reasons Pred and Reaper are so successful is that everything as actually very simple and proven from airframe and engines to weapons and avionics (except the sat link).
UAVs can't do everything, though - there are times that manned aircraft are better, but generally speaking long loiter surveillance is perfect for UAVs and these days strike missions require long loiter more and more, hence the additional utility of Reaper in Afg.
As an aside I saw somewhere recently (can't remember where but it was open source) that the Watchkeeper buy was costing £37 mil per aircraft including systems, support/running costs etc and at about the same time the 5 additional Reapers worked out at £27 mil per aircraft with the same factors included. Of course you have to be very careful with these numbers as I am sure the contracts weren't anywhere near identical, but it makes you think!