Getting a re-mark dos not guarantee that you'll get a better UAI. The best way to tell if it's worth paying for is by comparing your internal assessment results with the external mark, both of which should be on the letter sent by the Board of Studies. If they're markedly different, then you should follow it up witha re-mark. Again, there's no guarantee that you'll get extra marks because of it - you could have just had a bad run in the exam - but it's more likely thn if your internal and external marks are close together.