I see no problem in declining the events you can't afford to attend. And please don't offer to step down just because of that. I had two bridesmaids that couldn't attend certain events and it was no issue. I would have been a little insulted if they offered to step down simply because they lived too far away for the shower/bachelorette party.
And as an aside I dont think there is anything wrong with having a destination bachelor/bachelorette party. Just because the OP can't afford to attend doesn't mean it's a hardship for anyone else and I don't think a bride/groom is obligated to make plans based on the lowest disposable income of their friends.