Hi there, you need to charge BIG TIME!
Get the company to send you through a brief of their venture, their product and exactly what they would like you to provide advice and expertise on.
Then send them back your quote for the job based on this, the potential impact of your expertise on the outcome and success of the product venture with a watertight disclaimer that covers your arse against any future repercussions.
Remember that they are seeking your expert advice and as such you should honour that by charging BIG!
A daily rate of $1500 - $2000 AUD would be a good start but if it was me and someone wanted me for 2 days I would probably charge $5K AUD at the minimum.
Now someone with a bigger view of themselves then me would charge more than this.
So the question is...what do you think you are worth?
Go get em