Having read your words at the starting of this thread my first emotion was: “I am VERY pissed!”
But circa one and a half second later my thoughts completely changed. “Kerkythea” is neither OpenSource nor under some free licence model like GPL, BSD, Apache or something. It is a closed source project that benefits from the spare time of you main developers. And Kerykthea is a wonderful render engine. So why should the developers do not earn money from it? Simple: There is no reason why you should not professionally sell your own product. So at last I fully appreciate this step.
You should well consider your professional environment where you are placing “Thea Render”. There is of course “Vray”, there is “Maxwell Render” and a couple of other well developed render engines which are the choice of the – so called – professionals. I don't see any major advantages of Kerykthea/Thea Render compared against the other render engines. Given the background of your renderer it is clearly is the choice of hobbyists and semi-professionals. This should be reflected in the price range.
My very personal opinion is the following: Setting it at around $200 would be an instant buy for myself. Period.
Settling it in the $400 - $500 range would result in thinking more than twice. In this case I would consider “pmG messiah”, which has a wonderful renderer and is also a fully professional animation package. Also – maybe in some cases – the professional version of “Carrara” might come in place. The render engine quality is nowhere near Kerkythea but it has a modeler and it has animation capabilities and it is well backed by either it's owner company and by third party developers.
Setting the price range even higher might prevent it from having success from the start on. For example: Even myself as an enthusiast hobbyist is considering buying the upgrade option of “Maxwell” right now. But I would buy Maxwell because of it's professional recognition in the industry. That's missing by Thea Render.
A fast decision and clear announcement of how much we must pay for the final Thea Render would be very appreciated.
Having said all this I would not be sad if Kerkythea would die and you concentrate on the professional Thea Render.
All the best,