Well, in May it was;

Train tickets - £20 return. I got lucky somehow.
Underground tickets - Probably about £20 across the whole weekend.
Hotel - £60

Then my Hermione costume was £107 total.

Theeen, I only think I spent around £30 on food over the weekend.

Then I spent £70 shopping.

So £307 total? I hope for it to be even cheaper for October but I doubt it xD

Really it depends on where it is. For example Minamicon was way cheaper than Kitacon because of location. As for cosplays, depends on complexity. For Expo cosplays, if I'm entering the masquerade then it's more likely a complex costume (unless it's for Performance or something) so it will cost sometimes even above £100 for one! Other cosplays I like doing are generally cheap, under £20 each.

Here's Ayacon (4 days):
Hotel - £57
Travel - Can be up to £45, depends if I drive or go by train (not buying an advance ticket, too late for that now)
Ticket - £45
Cosplays - £50 (4 costumes - that's not bad!)
Food - £30
Spending money - ???
Total: £182

I don't buy much merch these days. I only ever buy things that are cheap so I tend not to go above £30 these days. I also bring food with me to cons to snack on - saves me from buying lunch at the con. Drinks I also bring with me. Really £30 is an overestimate but I'd rather have more than less!

For Kitacon III I think I spent £237 at an estimate. My cosplays were more expensive because the majority were made from new fabrics rather than leftovers unlike Aya.

If I base this over October Expo Weekend, I would get the Train in Both mornings and return in the evening.

Cosplay + Wig = £ 215
Con price = £ 28
Hotel Price = £ 0
Train = £ 58
Spending Money including food = £ 220

Had been planning to stay overnight, so I would have allowed about £ 80 for hotel. They are now a lot more than that, and it's only £ 29 per day for a Day Return Travelcard, so that's what I'm doing.

Usually I spend about £400-£500 I break it down into estimated costs.

£100 per outfit (Usually two outfits so £200)
£100 for a hotel
£200 spending money

I don't really need to sweat over travel expenses, because it's pretty easy for me to get to London (About 30 mins-1 hour on the train?)

But yeah, I'm probably going to struggle to make that this year unless I'm lucky enough to finda job pronto!