When you accept an invitation to someone's bridal shower or fundraiser, what obligation, if any, do they have towards you?

Someone invites you to an event that costs you money. At a minimum, you have to pony up for a donation or a gift. There may be incidental expenses related to travel or clothing, etc.

What do you expect back from them? Do you expect them to accept an invitation to your wedding? Do you expect them to donate to your child’s school fundraiser? Do you expect them to give to your charity fundraiser that you are organizing?

In general, how do you see obligations between friends, where those obligations can be expensive. Do you feel like you have to do what you’re supposed to or risk losing a friend? Are these obligations less serious or are your friends more understanding if you can’t do it or won’t do it? Do you think the rules are different depending on how well off you are? What other factors change the rules of social obligation?