(grining) Believe it or not, this is also an awkward question for most of Chinese.

But at least, the hong bao should not be less than RMB 200. That is a base line, which also you can consider as the normal amount in Beijing, I think.

the normal amount = RMB 200(in Beijing)
close friends = up to you, probably more than RMB500, if for me, I probably will give more than RMB 600 or 800, I mean that one is really a close and good friend.  
friends = RMB 200 or RMB 500, don't need to more than RMB 500
close colleague = if this colleague is just like your friend, the answer could be as same as the last one.
acquaintance = 200 or zero.

My experience:

1. My mom told me once that if someday my dad's best friend's daughter, whom I could call elder sister, would get married, my family would give RMB 1000 or 2000.

2. Several weeks ago, my colleague just got married, she invited us to join her ceremony, but I had a business trip, so I had to miss that, then I didn't give her any hong bao. We are just colleagues. And other colleagues like me also didn't give any hong bao. She can understand that. But if I attended that event, I must and should have given, and the amount would be RMB 200 or more.

Tips:

1. Be careful, DO NOT give amount of 300, 400. The pronounciation of 3 in Chinese just sounds like 散san, of 4 just sounds like 死 si. Both of them may imply UNLUCKY. To the contrary, 600, 800 imply the good and rich meaning.

2. You may ask people who will go to the wedding ceremony what they will give. And the better way is that all of you give the same amount. When you know how much they give,  you just need to follow them.

3. You also can send the gift to the newlyweds. The price of gift will be equal to 200 or just around 200, less or more.

Wish you luck! haha