Any point in time at which you learn some new material fact about the event, is a new point at which it's legit to decline. So if they announced the event six months ago, but told you only a few days in advance that it'd cost (a lot!!) then that's a point in which you can certainly decide to decline.
I agree with the above answers, that you should politely make it clear that it's about the money, and not make up some sick-aunt excuse. If they push back that they committed resources based on your earlier positive response, then the answer should be along the lines of "Sorry to hear that, Boss, but I only learned yesterday that there would be a cost, and it's just not something that's in my budget right now."
It is deeply manipulative of this workplace to set you and your co-workers up like this, and you have every right to be offended: you are the victim here, not them (assuming you've told us the full story). In fact as you discuss it with them, consider using phrases like "I understand your difficult position" and "I'm sad to hear that" instead of "sorry".
You may need to start making long-term plans about your commitment to this outfit.