I apologize for being cynical while you are trying to develop a new product.
I said what i did because i'm cheap, like most Archicad users are. We've spent a lot of money on an overall excellent application that has many second-rate features, many of which are excoriated on this forum. I initially mis-spelled "excoriated" as "excoritated." Besides inventing a new word, this describes the feeling, say, of being told that you are getting LightWorks included and discovering it is the old LightWorks not the new LightWorks because that would be too much work. Excoritated.
For a fair answer, then, to get real-time rendering in Archicad is worth thousands, but only to those few who can apply it and aren't resentful of Archicad's weakness in the rendering area. So, as a marketer, you'd want to assess the market SIZE rather than the unit PRICE. Relying on my experience giving illustration seminars and selling my LightWorks book, you'll be alarmed and disappointed to see how few Archicad users actually make photorealistic imagery of any kind from their models.
I don't know the reason but i suspect:
-- it takes a lot of time to properly set-up a rendering scene.
-- even with your promise of real-time feedback, the technology learning curve/scene refinement curve is immense.
-- users don't have the artistic sense to make satisfying images because they haven't been to art school, film school and professional photographer's school, aspects of which all apply when making architectural imagery.
The buyer would be making professional images (?) This means that your product would need to access superior forms of entourage that Archicad currently doesn't - like animated figures and foliage. The things that Cinema 4D can use, for instance. In part, users export their models to place entourage or engage animation features like wind.
And your product needs to be fast on a real model. A "block of flats," say. Those of us who have been taken in by certain oleaginous (look it up) Hungarian-accented voices demonstrating speed when rendering a mocked-up shack to then discover the grinding mess a real project can be are understandably suspicious of speed claims.
But if you have a product that can seamlessly plug-in to Archicad and give real-time rendering the quality of Cinema 4D with sophisticated material and lighting definitions, why stop at Archicad? There's thousands (hundreds??) of Cinema 4D people out there using outside Renderfarms to do their animations. Now, there's a market!
I'd be happy to brainstorm with you about this.
Dwight Atkinson