Even if it were a weather balloon, the sensors on it would be insane and worth more than the missile. A spy satellite's sensors? Those lenses are filthy expensive.
Size matters, even just considering raw material costs. The fabric is not a cheap coarse cotton garment that would otherwise have become a pair of Levi's. Its full of Helium. On top of that the balloon's size makes it a difficult structural problem.
Not saying you're wrong, but... what is your basis for saying this?