I, personally, would not say I couldn't afford to go. I'd simply refuse the invitation without an excuse. And I would do it in a nice note or card, congratulating the couple on their marriage. Something like: "Mr. Froogs and I regret we will not be able to attend your wedding. Thank you for thinking of us. We will be thinking about you on your day, and wish you a lifetime of happiness. Best wishes to all, Froogy."