Answer:
The ideal context for asking your parents for money to buy an expensive new outfit could be the conversation about an upcoming event, where you are supposed to be dressed well and formally. Under such circumstances, asking for money to buy a new outfit would be an ideal solution, as the event you are going to attend, requires from you a certain outfit, which you will buy for that occasion.
On the other hand, you could discuss buying an expensive new outfit with your parents as a reward for some good grade you got on your exam. Therefore, once you get the grade, you should mention it to your parents, and it would represent the ideal context for asking them for the money you need.
