Checkmate. Go away. Krugman is a complete joke.
You asked me once we blow all of our money repelling the aliens, then what next?
I say, we would need to identify the aliens and their biology, what their goals were and figure out if we can prepare for or combat additional attacks and when they may occur based upon their position in the galaxy. (we would also need to figure out their origins for this obviously.)
I thought that was an adequate answer to your question.