Etiquette question - Is it bad to turn down a hen do invite?

Here's the situation:

One of DH's best friends is engaged, and his fiancee has invited me to her hen do.

At first, I was happy and excited for even being asked, since I don't really have close female friends so I was flattered that she thought enough of me to ask me, and I thought it would be nice to do some female bonding. However,

1. It is at a music festival, and I have no interest in going to a music festival.
2. Tickets are expensive.
3. DH and I have been trying to save money, and we haven't had a holiday ourselves in ages.
4. It would be the first time in the 5 1/2 years since I moved to the UK that I spent a night away from DH. (I wouldn't mind if DH went to the stag do, because the groom is someone DF has been very good friends with for many years.)
5. The bride drinks A LOT and I assume many ofher other friends drink like her. I don't mind spending an evening with her at the pub when she's very drunk, but I don't know if I could deal with it for a whole weekend. (I'm a moderate drinker - If I go out to the pub, I'll have 2 drinks maximum.)

So I really don't want to go, but I don't know if saying no will cause a social problem.

Another issue, which I don't know if is relevant, is that the couple are having a tiny registry wedding and then a big non-official wedding in  a castle. DH and I are invited to the castle wedding but not the registry wedding. DH is upset that we weren't invited to the registry wedding, since he and I had a registry wedding (that was our only wedding) and the groom was a witness. The groom was also at the registry wedding of DH and DH's ex-wife (not sure if he was a witness there or not.) DH has told the couple that he is upset about not being invited.

She has asked me via Facebookand I haven't responded, so I haven't had a person to person conversation with her about this yet.

Advice?

If you genuinely can't afford to go, then I would hope that she'd be understanding. You may not be the only one to back out for financial reasons.

I think it's completely fine to not go, especially if you don't know her very well. I'm sure she's not expecting everyone to be able to afford a music festival (and holy crap are they ever expensive). Say something like "sorry I won't be able to join you for your hen weekend, but we're really looking forward to celebrating with you at the castle". Done.

I totally agree with squirrellypoo.  If she is upset that you aren't going (which hasn't happened to me when I've turned one down before), you can always say that you only have the budget for one of you to go and since your dh is closer to her df, you thought it would be nice for your dh to go to the stag do. 

Thanks. I just wanted to make sure it wasn't seen as a personal insult to the bride-to-be, particularly since DH has been whinging about not being invited to the register office wedding and treating it like he's been personally insulted.

I turn down invitations to hen-dos more than I accept them.  Because of our work schedules, DH and I spend much of our quality time together on weekends, and we are also trying to save money.  No one has ever batted an eye. Maybe you could make sure to catch a cup of coffee with the bride sometime between now and wedding?

I turned down a hen-do invite earlier this year in much the same circumstances as you... It was a lot of money that we couldn't really spare and I wasn't up for a drunken weekend with people I didn't know very well.  I just offered to take the bride to lunch some day to catch up instead and that seemed to satisfy everyone.  And as it turned out, there were a lot of others who thought the cost was too much and also didn't go, so it wasn't just me!

I've turned down a couple in similar circumstances, one was a weekend at a spa and I just couldn't justify the cost, seriously do you need 3 days of expensive pampering?!

One was in some horrendous sounding Spanish resort which made me want to weep.

Both times I made sincere apologies and caught up with the bride at another time. Hen do's have got ridiculous.