I told my stepsister I'd ask about this. She's a BM in a wedding where the MOH wants to throw a weekend event in a city about 100 miles away. This will be 2 nights hotel, dinner, a spa day etc.
Well, the cost to your stepsister will be more than that--there's transportation to and from the city. Even if she drivers herself, there's gas to pay for, or a train or plane ticket, etc. Then she'll be eating three meals a day for at least 2 days, and one of them, the dinner, sounds like it might be expensive. The "etc." also would concern me, if I was expected to attend.
Because bridesmaids are usually expected to attend the bachelorette, if not throw the bachelorette, I do think that the MOH should have run the plans by them before making a final decisions. At the very least, the MOH should have asked the bridesmaids about their budgets for the bachelorette. Since she didn't, the email announcing the plans should have included that information: cost of the shared rooms, the name of the restaurant where the dinner will be so that they can check the menu and see if they can afford it, the cost of the spa day, and the cost of anything else that's been planned. If the plan for one night is going out and hitting as many bars as possible, the bridesmaids need to know that.
If the bachelorette was simply a night out on the town in their local area, the bridesmaids would be able to estimate the cost a bit--they'd know the restaurant and bars they'd likely go to and what the prices would likely be.
With an out-of-town trip, the costs need to be explained in full. It would be a huge damper on the weekend if a couple of the attendees were hit with unexpectedly high costs, and had to continually back out of certain events, or were constantly ordering/doing the very cheapest thing available.
Your stepsister should email the MOH right back and ask about the costs of everything, including what the bridesmaids are expected to cover for the bride.
I do think it is rude of the MOH to plan an expensive weekend without input from the people who will be expected to be there. (By expensive bachelorette, I mean anything other than dinner out and some bar-hopping. Anything that requires hotels starts to get expensive in my book, because that usually means restaurant meals and travel expenses.)
It really doesn't matter if the bride likes the plan or not. If her friends and wedding party can't afford it, the plan never should have been made. The MOH should have consulted with the others, and come up with a plan that everyone knows they can afford. To make the plan first means that some of the bride's good friends may not be able to attend. The bride had better not be upset about that.
Sometimes you have to choose between having the people you want or having the event you want.