The best response I've got is probably "Well, hiring me will be expensive, but not as expensive as hiring the wrong applicant instead". However, for obvious reasons I wouldn't recommend actually using that one under most circumstances!

Being serious, anyone asking something like this is probably just expecting you to kiss ass without being too obvious about it. If you really could make an exceptional contribution, perhaps because it's a field you genuinely want to work in after doing some related research or project during your course, by all means mention that. Otherwise the best thing you can do with a question like this is be polite and constructive. It might help to pick up on specific things you could offer that have been touched on earlier in the interview or at least mentioned in the job spec. If it's still early in the interview and you haven't got to that sort of discussion yet, try to turn the conversation around and discuss what they're looking for and then how you could help.