We're clear that this pricing is wildly irrational, right? $30 for a half hour is, literally, not what it would cost to assess someone's skill at performing haircuts. Many of your customers, who after all have hiring authority for engineers, can do the math about what your interviewers' market wages and chargeout rates must presumably be. Those rates make these rates look like a joke. That quickly sets your customers to thinking "OK, there must be a catch, because clearly one cannot sustainably pay a CTO $30 per half hour to do schedule-in-advance work with a half-hour minimum? What is that catch?"
None of the answers your prospects will dream up to that question will induce them to give you their business.
Companies like Gerson Lehrman Group (GLG) pay experts $750 per hour for one hour of talking to someone on the phone.
I highly recommend patio11's newsletter :)