If it is only for a 90 minute session, I think you are better off asking a favor from someone you know, or from someone who might do it for free. Yes, some people might consider doing it for free, especially with vested interests. For example, if he can promote some products or services of his own to professionals for 15 minutes of that 90 minutes, or provide an extension for the promotions.
This should eliminate costs. If you can give a win win situation - it would be very productive.