"The correct answer is that now I'm no longer employed by you, the consultancy rate is (5-10 x previous rate) and you would be happy to help, but given the circumstances, the terms are payment in advance."
It's actually better to decline.
If you start demanding high rates they may sic lawyers on the case claiming blackmail.
I've seen it happen. Wait for them to make the monetary offer and give them time to worry you'll refuse.