Hen and Stag nights / weekends are big business, and prices are, apparently, increasing. There's loads of companies willing to take your hard-earned cash and arrange the venue - complete with T-shirts, funny head-gear, stretch limo's, and strippers for both the hens and the stags.
I wouldn't touch them with a barge-pole.
Your niece should refuse to go - that's a ridiculous amount of money to spend when she's getting married herself shortly. Of course the bridesmaids will put pressure on her - but she shouldn't feel guilty, they'll all still have a good time without her, and she'll have that money for her own wedding. Teach her to say "no" - not easy - but you have to learn not to let people organise your life for you!
Marydoll's DIL's hen do sounds nice!