I tend to agree with most we have always given about $50 for a 'friend'' and $100+ for a close friend or relative.
I travelled interstate with a 2mth old for one of my oldest friends weddings, my accom, car hire, flights and food would have cost nearly $800 and he knew $$ were tight so he told me NOT to put in anything but i still threw in an extra $50 with my dad & stepmum.