This question highlights one of the most important aspects of the GMAT: Correctly interpreting the question. If you consider that only 320 bulbs were used to light the gym, and each was 1$, then it's very easy to pick 320$ here. However the question asks how much money was spent on illuminating the gym, which includes waste since that money was spent and not recuperated. Sometimes students tell me: "I don't think this question is clear". Unfortunately, no one is there on exam day to help clarify the issue for you, so you have to figure out the intent of the question on your own. Practice will help a lot, but the stress of answering questions in ~2 minutes as well as tricky wording will always leave room for misinterpretation and missed points on this test.
I highly recommend rereading the question for 10-15 seconds before clicking on submit, particularly for Problem Solving questions in math. Whatever the right answer to the question is, you can bet there will also be the right answer to a misinterpreted question among the choices.
Hope this helps!
-Ron