Be honest. One reason that I don't believe has been touched upon is this:
There might have been discussions about whether going with a more expensive event, and asking employees to pick up the costs, would force workers to decline and be excluded. The fact is, it is the cost that is making you decline. Let them know that. Perhaps in the future they will opt for smaller, simpler, less expensive but more inclusive events when planning.
A simple -
Unfortunately, the cost-sharing is out of my budget range for a social event like this.
should suffice. If they want to pry further and find out what your personal "squeal point" is, financially, simply state that this is personal information, as well as situational, and you decline to share it.