How can I tell my best friend I can't be in her wedding?

I live on another continent and my best friend is getting married in just over a year. She asked me to be a bridesmaid and I said yes,since we have been friends since grade school. However, now looking at my budget & expenses (plus my own plans to get married and buy a house), I realise I can't afford it. ($1500 airfare alone) How can I tell her this without hurting her feelings and getting into a huge fight? Is this possible? HELP!