[w0de0]

It's one balloon, Michael. What could it cost, 10 dollars?

[phil21]

A large Mylar party balloon with helium? Yeah probably about that depending on where you are and the balloon type and size.

[SideburnsOfDoom]

Yep. It seems like for this application you'd want a larger one, a few feet across, with a nice shiny metal foil coating for the radar to bounce off. So, not a $1 balloon.