You don't need to be the one getting married to say "I do" to substantial wedding bills.
The average wedding guest will spend $673 per wedding this year, according to new data from American Express. That's up 13 percent from last year and 21 percent from 2013. (Tweet This.)
Scarier, that tally is just to be a regular guest—watching the couple exchange vows, enjoying some cake and (reluctantly) hitting the dance floor for the electric slide or chicken dance. Members of the wedding party are likely to spend even more.
Most of the total stems from travel costs to attend a destination wedding, or even just one that isn't in your hometown. Amex estimates guest spending at $225 for airfare (up 125 percent from last year), and $170 on a hotel (up 86 percent). (Check out the video above for some of the other cost factors that guests should consider before RSVP-ing "yes.")
Read MoreWhat that perfect wedding will cost you
It's not the bride and groom's responsibility, per se, to consider their guests' tab in the course of planning. "We really talk to brides and grooms … about how you have these three, sometimes four factors that are really paramount to making the decisions about your wedding," said Lizzie Post, co-host of the "Awesome Etiquette" podcast. "After that, you have to let the chips fall where they may." If the couple's priority is having all their friends and family there, she said, their choices of location and other details are likely to reflect that (i.e., it's less likely to be a destination wedding or a black-tie affair that requires tuxedo and gown rental.)