Your friend is getting married. Hooray! Now pay up. On average, millennials spend nearly $900 for every wedding they attend as a guest, according to numbers crunched by American Express this week. In comparison, the average American wedding guest spends 27 percent less, around $703 per wedding. That number accounts for an average of $205 spent on airfare, $166 on attire, and $69 on child or pet care. Millennials, though, break the bank by spending on average $893 per wedding or, if they're a part of the wedding party, $928.