While I don't think a weekend bachelorette event is inherently rude, in this case the MOH and the bride erred by getting their hearts set on something without consulting the other invitees, especially if they're expecting people to pay their own way. Even if the bride's wealthy aunt was going to pick up the entire tab or something, it's still a big time commitment and the activities might not be to everyone's taste. So, there should definitely have been discussion about the plans first, with a consensus reached by all the bridesmaids and the bride.
If MG wants to decline, as the other bridesmaid did, I think she's perfectly fine to do so. Fine etiquette-wise, anyway; I can see the MOH and the bride being really disappointed, and even though it's their own fault, the friendship could potentially go downhill.
If MG is more inclined to go, there's no reason she has to be blind to the costs and itinerary. I would email the MOH back and say, "Hey, sounds like fun, but I need a better idea of the plans to see if it's in my budget. How are we handling transportation there and back, and around town? What's the cost of the hotel room, and are we to share or get our own? What are you thinking for meals each day? What other activities will we be doing, and how much should we expect them to cost? Finally, are we expected to kick in anything for Bride, and if so, how much?" It's a lot of questions, but the MOH should have answers to all of them, and should in fact have told them to everyone already. Also, keep in mind that some costs may change if other bridesmaids drop out--a hotel room split four ways may drop to being split only two ways, for example.
If the MOH doesn't answer satisfactorily, or the number is too high for MG, I think she should decline cheerfully. "Sorry, that's not in my budget right now. I hope you guys have a great time and I look forward to seeing pictures when you get back!"