Many friends asked me that many times, how much to put into red envelope while attending friends wedding? It really depends how close you are to the person that you attend wedding, what is your and their social status, or if you are related in some way or another.
The idea is that you try to give a sum that have those numbers: 2, 6 and 8, and make sure you avoid 4. Six and especially 8 are considered to be lucky numbers, while four sounds like 'die' in Chinese, so people avoid it.
If you are not close to the couple that are to become officially husband and wife, you should at least offer enough to cover your food and reservation - NT1200, NT1600 or NT1800 are good sums if you arrive alone or with maximum of one person.
If you are closer friends, start with at least NT2000 and up, depending how close you are. Again, make sure you avoid sums that will have "four" in it. Just one word of advice, giving too much might be also not advised. Later when it is your turn to have your own wedding, your friends will feel responsible to give back to you the same amount, and often - even more, simply to 'save their face.' If you already had wedding, and for some reason they were not invited - you can feel a bit more generous.
Last part, but not least - the infamous visitor book. You will have to put your name in it and the amount of money you put in the red envelope, for everyone to see. Ain't that sick? :).