What is the proper etiquette for gift giving for a wedding?

I have been invited to a wedding in which I have already attended the wedding shower and bought a gift. Is a gift expected again for the wedding? There was a gift registry enclosed with the invitation and there is nothing on the list less than $100.00.
I am unemployed at the moment and really cannot afford another gift. So can I go without a gift or should I just not go since I can’t afford to get another 100.00 gift?