What was the economy of Prussia like during Frederick the Great's rule?
Friedrich der Große wasn't called "the Great" for no reason. When it comes to the economical side of his greatness, the economy ran fine when it comes to his whole tenure.
Seven Years War: During the Seven Years War however, the economy sort of collapsed for a while, the state budget was 7,000,000 thalers, 5,000,000 was spent primarily for the army of 200,000 men (as Prussia had a military culture). So Prussian funds ran extremely short when they were getting invaded by Austria, France, and Russia. The lands got ravaged (Altmark, Silesia, etc.) but the economy itself remained smooth. Other than that, there was a level of immigration towards Prussia which boosted the economy (pre Silesian conquests Silesians moved towards Prussia, Lower Germans aswell, even French, all served in the ranks of the army by the way).