Gross domestic product (GDP) is the monetary value of all the finished goods and services produced within a country's borders in a specific time period.
Recent survey findings from RetailMeNot show that this season's wedding guests plan to spend $195 on a sibling's wedding gift, $159 on a best friend, $63 on a coworker and $45 on an acquaintance. Yet other sources say somewhere in the ballpark of $75-100 for a coworker, and $100-125 for a friend is appropriate.
I agree that your gift should not depend on how much it cost the couple. We had about 100 guests at our wedding and I would say the average gift we received was between $100 and $150 (closer family and friends gave more).