Well the fact it is going to cost you £100 is something of a drip feed.
I presume many other posters were thinking the same as me, that it was local friends and the party was somewhere nearby.
OP you clearly don't want to go, so don't go. It's not a big deal. It is certainly no skin off the nose of anyone on this thread.
Stay at home and sulk, as a pp said, and accept that if you don't go to things when you are invited, then people will stop inviting you. Which is fine if you don't want to socialise, although seems at odds with some of what you have previously posted.
If you consider attending a party is "putting yourself out", then it probably isn't for you. Personally, even when I get an invitation for something that would be nigh on impossible for me to get to, I still think "Oh, that was nice of them to invite me" rather than "Why should I put myself out for you" <-- that is such an odd mindset.
The way I see it is that someone thinks enough of you to ask you to come to their 'do' and that is a nice thing. If you want to go, then go. If you don't, then don't, but don't try and make out you are doing them a favour by gracing them with your presence.