I guess it depends largely on what kind of event it is. Promotional events (think Lego, or something for Lucasfilm like Clone Wars, or a new Star Wars game) you might end up getting freebies, or discounts, or some promo goods.
Children's birthdays, charity events, small town festivals... well, I'd say make sure you won't at least dehydrate and die
That's from my experience. Free entry has been a given at most events, especially if we got invited in the first place - of course you can't expect to show up in plastic at a big Sci-Fi con uninvited and demand free entry just because you're so damn cool