A lot. It's really hard to know exactly, but a good place to start is the Marshall plan. In today's currency exchange, it gave about $150 billion total to Russia, France, UK, Japan, and other European countries. This money was used to repair villages and cities that were destroyed in the conflict. This was only the beginning of reconstruction, however. Most countries didn't return to pre-war form for 30 or 40 years.

The exact dollar amount spent to fix the damage of WWII is almost incalculable, but my estimate would be hundreds of billions, maybe even trillions. The war was unimaginable destructive. Hope that helps!