If it's not a local wedding for *most* people, can you do the events the days leading up to the wedding? My older sister lives in SC and got married there. Three bridesmaids came from MD, where the rest of the family lives, and one came from Boston. We had the bachlorette dinner on Thursday night, the shower on Friday, and she got married on Saturday. So it wasn't two trips for everyone.
My younger sister is getting married in May. Our older sister and I are both matron of honors, and we are doing both events in one day so that those traveling from out of town (older sister, aunt) can be here for both, but only come one time. Maybe that's an option you can put to them?
Also, if you simply can't afford it, I'd let them know you are really sorry but the situation is what it is. Don't feel bad.