The question is, "How much money did you spend because of attending the interview?" The writer is asking about costs/expenses that the other person incurred due to the interview itself. (So if the person had to pay for a taxi to get to the interview, that would count as an expense. If the person decided to buy a gift for a friend before the interview, that would not count as an expense, since it is not related to the interview.)
The phrase "in attending" means "because of attending." You can think of it like "in the act of attending" or "in the process of attending."
If the writer were saying that the other person should give information about expenses during a certain meeting, s/he would need to write the sentence differently to clarify that meaning, probably by putting "during the meeting" (or a similar phrase) at the beginning of the sentence, not the end. "In our next meeting, please detail any expenses you've incurred."