One thing I wonder is if the kind of people that would take up an offer like this have the means to pay for it. As in: do the "idea guys" that can't find technical guys have enough money?
One angle you might want to consider is the sanity check - going over the technical merit of an idea/early stage product/architecture. This could be for both the idea guy and technical guy. A company that is a bit further along would have more cash.
And for those on about the rates: that rate for two hours is not bad.