It is easy to define the ang bao rates for wedding. All you need to find out this how much the couple paid for the wedding banquet per table and then divide by the number of pax per table. All these wedding banquet rates for each hotel can be easily found online. How about ang bao rates for housewarming? Is there a market rates for it as well? The answer is yes and no.
First, it is important to find out if your host catering any food for the housewarming, if there is lunch or dinner catering buffet, you can just pack based on the number of pax attending the housewarming.
The typical price range for catering is about $30 to $40 per pax. If we assume $30 per pax and you are your partner is going, you can pack around $80 to $100 for two pax. If you are going alone, probably something more than $50 per pax.
If there is no food provided, it is just an informal get together you can just pack a small ang bao as a token or not pack at all. If you are going with your friends, best to check if what they are bringing or if they are packing ang bao. It is always polite to ask your host if they need you to bring any food?
Alternatively, there are some who prefer to give gifts instead of angbao. There are some who will just buy a cake or bring a bottle of wine instead of coming empty handed.
Honestly, there are no fixed rules and guideline, it is really up to one generosity.