I arranged it with their agreement
Do you mean that you all mutually decided on it or that you personally suggested it and then went ahead with the planning after they nonchalantly said "Yeah, why not" ?
I'd test the water and say you might have to cancel it, as you can't afford to pay for it upfront.
If they seem happy with this, it means they were never really that bothered and so the holiday could have been very awkward, maybe with them resenting you for encouraging them to spend their money on it.
Assuming that they do want to go on the holiday, this may spur them on to do something about actually paying for it.
It massively annoys me when people claim that they can't pay their own share (when due or indeed ever), but they assume that you can and are willing to pay everybody's.
I agree with PP that, if they don't pay up before the holiday, when it could still be in jeopardy, they're very unlikely to pay when it's been and gone and nobody can take it away from them. It's very easy to 'forget' to pay after the event - a bit like when people sign up to a 1-year car insurance policy, agreeing to pay in 12 instalments, crash the car in the first month and then get outraged when they're still expected to make the payments for the annual policy that they signed up for.
Except it's even more outrageous when they expect their friend to sub them and not a massive insurance company.