To think this is too much for a 30th?

Right but why is a 30 year old going to wait until 50 or 60 before celebrating their birthday as a big occasion? Like that’s potentially twice their life to date? They might not even make it to 50 or 60, seems fairly joyless to not expect them to do much to mark the occasion.

How does she get away with not reciprocating? Surely the others in the group notice that too?

What do you get out of this friendship? Because so far, it doesn't seem like much. Friendship length does not determine quality

Sounds like you pretty much hate her so best you don’t go!

Sounds like you friendship might be running it's course. She's organised something completely that she wants, which is fine, but is making that the focus for all your birthday celebrations this year. The way you say she wouldn't reciprocate any of this and would make it known you have slighted her if you didn't go is indicating there might be history with her always being the centre of things. Is this irking you a bit. Do you think this might be a tipping point for your friendship.
Make the decision for you and if she gives out about it then use it as a time to rethink what you are getting out of the friendship.

Not a chance I would go with a large bunch of people I don't know.
You don't owe her a weekend where you are making up numbers.

Thing is in Edinburgh you are at your destination once you get off the train. Italy also means transport to and from airports and sourcing food and drink. I think there will be more hidden costs. As well as all the extra travelling time.
Which is fine, but I’d be disappointed about how short a time it is for all that money and travel.

I would go because I love Italy, and £450 is affordable for me.
If you don't want to go for whatever reason, just make your excuses and decline the invitation.

does she see herself as a queen bee of the group? Rather rude re your birthdays!

Life is short and shit. I barely have a low now with little kids. I say celebrate anything whenever you get a chance.

She can choose to celebrate her birthday however she likes. Just like you can choose not to go if you don’t want to or can’t afford it.

I know I could say no but it would be taken as a major slight. It’s all of our 30th next year, and I know that this will, for her, be considered to be the group’s major celebration and she’ll barely acknowledge ours.

Sounds like you aren't that keen on her anyway, so nothing lost by saying "Thanks for asking, but no thanks" or "I can't make that, but hope you have a great time" or something similar.

Anyone who is offended by people not being able to make what would be a big ask, is not really worth worrying about in terms of them being upset.

Absolutely nothing wrong with her choosing to do this for her birthday, but equally, no-one can seriously expect 20 people to want to
a) go for a short break to Italy
and
b) go away as part of a group of 20 people
and
c) go away with people they don't know well or don't know at all
and
d) have enough annual leave and be able to use the annual leave then
and
e) have the money to spare (because, lets be honest this is going to end up being closer to £1K than £450 pp)
f) even if they have the money in their bank account, choose to prioritise this break over other things they might be saving for
so if she takes offense at any of the invitees not being able to go, she doesn't sound like someone I would be worried about offending.

Oh, that's totally valid, sorry I missed that point. No, aside from one or two old friends whose families I'm close with, I wouldn't want to party with parents. That would be weird.

Is she married? Has she ever been celebrated like this for anything else? I think it's not unreasonable to want to celebrate your big birthday like this and 450 is not unreasonable.

Is it a case you can't afford it and would have to give up a family holiday or just you don't want to do it?

Has she gone to events for you? Celebrated your wedding etc?

Without knowing all of this I haven't voted but I think life is about celebrating our friends and family and it shouldn't always be a wedding or a baby that we do that for. We also need to show up in ways we'd want them to show up for us.

Sounds like a fun trip and it's not OTT expensive. So, it's up to whether you'd want to go. Enjoy time with your friends!

Myself and all my friends are hitting the 40 stage and we all have small children, and have more constrained budgets, so there haven't been any big celebrations. Certainly no trips away! Enjoy it while you can and your still young :)

Why? That makes no sense. The only 'decade' birthday which is less than the others is 20, because it is bracketed by 18 and 21. The others are all equally milestones.
My 40th was a huge bash. Most 50ths are not IME. The only place where 50 is a big deal is the Netherlands, where turning 50 is called an Abraham birthday for a man and a Sarah for a woman, for some reason. It's A Thing there, but that's the only place I know of where that's the case.

I've said YABU. Not because it's at all unreasonable to be unable to afford something, or just to choose not to spend that money. However, YABU to say that it's a big ask for a milestone birthday, but not for a hen do or anniversary. If anything, I would be more inclined to pay a large sum for a birthday than to celebrate an anniversary!