Invitation - financial embaressment

Much better to tell her that you can’t afford it than to just not turn up. She’ll understand. Ideas about sending a special gift for or after the event will make you feel better and show her you care.

As others have said honesty is the only way with this . I would think that if she is a close friend then she would have some idea of your financial limitations? The overall cost is the same as a holiday would be.
It may be that she she is not realistically expecting you to be able to go and your declining the invitation will not upset or surprise her.

Goodness, our friends put us in some rum situations don't they?
All you can do is as advised above; say thank you for the invitation but it's beyond the purse strings at present. With the dire warnings on the news everyone will understand and, I suspect, secretly admire your honesty and bravery in saying so.
Can you arrange for a surprise gift - eg flowers/some special favourite food - to arrive at her home when she gets back after the party, to take the edge off the post-party flatness?

Oh I meant to add send her a card, flowers or some small gift if you are able to, to mark the occasion.

Yes, honesty is good for both of you. Sure, you'll both be disappointed but that's much easier to cope with than debt or money worries. Good luck. Been there. Many tshirts!

I think you have to be frank and just say, for you, it's unaffordable, I don't see why there should be any shame in that, everyone's circumstances are different. There are loads of threads over on MN, usually for hen parties where many say exactly what you have said. I think any friend worth their salt would know that this is not you snubbing them, you'd go if you could but you've outlined the costs and they are considerable. Only you can know your budget and what it will stretch to. I think I would couch it in such a way to suggest that if she is coming to your part of the world anytime soon then maybe you could both have some sort of belated celebration, In the meantime say you wish her a lovely time, hope it all goes well and you were honoured to be invited.

I would add that I don't think anyone has the right to be upset if someone turns down an invitation because they can't afford the expenses that will incur, obviously I wouldn't tell her that.

Just be honest and tell her that you cannot afford it .

If you can afford to send her a special bouquet and a nice card and she is a close friend as you say, she will understand.