Am I being unreasonable....

To say I can't justify spending £250 on attending my SIL (OH's sister) hen do? I only see her approx three times a year so we aren't exactly close. I don't know any of the other people going, and I wasn't planning on inviting her on mine!
It's about 400 miles away from me (Newquay) so travel costs, plus a night in a swanky hotel, plus a fancy dress costume, dinner, drinks and club entry all add up. I don't drink either and really detest nightclubs! I sound like such a party pooper lol.
I said I would go originally when I thought it would cost considerably less. To make matter worse OH changed jobs this month from one that was paid in advance, to one paid in arrears, so hasn't be paid anything this month! We are winging it on my salary, and I refuse to dip into my savings for something as non-essential (and non-enjoyable for me).
OH is going on the stag do, which is the same night, in the complete opposite end of the country to where the girls are going (Newcastle). But he is genuinely close to the groom and knows several other blokes going. So I think I was invited on the hen, because he is going on the stag and not
because she genuinely wants me there. I did say 'oh don't invite me, just go
with your friends' (fully what I intend to do). But I still got invited. She said it will be good for me to know peoples names so I can help move people in the photos.
Am I being tight or should I stick to my guns and say sorry but I have better things to spend my money on at the moment?
It's about 400 miles away from me (Newquay) so travel costs, plus a night in a swanky hotel, plus a fancy dress costume, dinner, drinks and club entry all add up. I don't drink either and really detest nightclubs! I sound like such a party pooper lol.
I said I would go originally when I thought it would cost considerably less. To make matter worse OH changed jobs this month from one that was paid in advance, to one paid in arrears, so hasn't be paid anything this month! We are winging it on my salary, and I refuse to dip into my savings for something as non-essential (and non-enjoyable for me).
OH is going on the stag do, which is the same night, in the complete opposite end of the country to where the girls are going (Newcastle). But he is genuinely close to the groom and knows several other blokes going. So I think I was invited on the hen, because he is going on the stag and not
because she genuinely wants me there. I did say 'oh don't invite me, just go
with your friends' (fully what I intend to do). But I still got invited. She said it will be good for me to know peoples names so I can help move people in the photos.

Am I being tight or should I stick to my guns and say sorry but I have better things to spend my money on at the moment?