I wouldn't even think about trying to figure out what they've spent on you and gifting it in return, it's not a business transaction! They aren't a restaurant you've gone to for dinner... just seems really odd to me to be honest.

I'd gift what you would have spent on them as a gift. For us we usually do $50 for friends, $100 for close friends and family, $150 for close family like a sibling.

Don't over think it, I know for most people I know it's all about having you there to celebrate and enjoy, not what you 'pay for the privilege' :P

Oh and we usually include a card so we can write some well wishes and such but you can always give a card seperately too which some do.