Its not a problem of how much Blender costs, but the license under which it's source code is distributed. Blender could cost $1000, but it would still be GPL. Everything that links to its source code (as an integrated plugin would need to do), must be under the same open license. But RS/Otoy insists on a closed source SDK for Octane.
Also you can't take the Blender source code, change the license (make it closed source), call it Blendur 3D Magnificent Suite
and sell it.