The problem is that hen do's are a party that the guests have to pay for.
Most parties with any implication that you should go, the host pays for the food and entertainment.(weddings, birthdays etc). You contribute by paying for your own transport/accommodation, which you choose according to your budget (I camp at all the summer weddings I go to and share lifts, others stay in a hotel, up to them and no one cares). You take a present of a value that suits your budget, or not, as you see fit.
If you are invited for an impromptu activity weekend with friends (or a meal out) there's no pressure to go if you can't afford/don't fancy it.
People feel pressured to go to a hen do as it's their friend's special day, but then they end up paying vast amounts for activities they don't want to do, with people they don't know/like, which they never would normally have agreed to, so end up feeling resentful.
I think if you want people to go on your hen do, if it's anything more than a meal/few drinks, you should pay all or most of the cost yourself. The exception being if you know that the entire group already know each other and would have chosen to do that specific activity anyway.
The absolute least you can do is tell them the full cost at the time you send an invite, and ask for a deposit to be sent with the RSVP - that way no-one has any nasty shocks, if people drop out you can use their deposit to prevent everyone else paying more.
And if you have a bridesmaid that doesn't know the other invitees, it's asking for trouble to let them organise a big surprise as they won't know which others are likely to drop out/hate spa days/are pregnant/etc