Thew biggest expense for the hit would be getting caught at it.
Suppose the US decided to take out the President of Sudan, or Robert Mugabe: if there is a hitman, he might talk, or if you kill the hitman, his friends or family might talk. There are passports and visas, and airport photos and weapons and a lot of other problems. And no, I do not believe that the hitmen have already been paid yet. They don;t keep them on retainer, like lawyers.
There is no problem with the robo-skeeter. It does its job and flies away and gets lost in the bushes.