If I remember correctly, the number was rather 30 billion Deutsche Mark. But still, it was a good deal for the Sovjet Union. Personally, I think that Gorbatchew was very aware of the pure financial state of the Sovjet Union and this was the ability to get rid of the DDR not only at no costs, but having all costs of relocating the military covered by the west. Still, he managed to delay the collapse of the Sovjet Union only by a few years.