The last time i attended my friend's wedding at Pan Pacific, I gave $100. Think this should be the market rate for most hotels. You can also visit the hotel website to check the cost of banquet and including GST and other costs such as cockage, you can roughly work out the amount to give, to cover costs.
For myself, I normally give $100 to $200, depending on the location and reputation of hotels. The classier the hotel, the more $$ i give. I guess it also depends how close you are to the groom or bride.
Hope the info is useful.