"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."
You're way too nice :)
Either I'm already there on consultancy rates, in which case it's my summoning cost is being met.
If it's from a previous workplace, then I start with q request for a months salary for even looking at the proposal, and about a months salary = daily rate (or weekly = hourly).
I am no longer surprised when people will throw piles of money at you to solve their shit, who only weeks earlier where bitching about paying you a buck or two more an hour, and how your skills where easily available in the marketplace.
I am still a little shocked at just how quickly they agree. I'm obviously not charging nearly enough....