Extrusion of gray-scale graphic

I know that one extrudes surfaces and splines, not graphics.
I know that graphics can be used as displacement maps to create additional geometry... so the subject line is not quite what I mean...
What I do need is to quickly build 3D solids based on irregularly shaped cluster of gray-scale rectangles. The outline of the cluster should create boundary of the solid in plan (top) view and gray-scale values of the rectangles should be used for relevant extrusion of that part of the solid on Z axis (height).

I realize that I can simply trace the graphic in Illustrator, extrude the resulting vector outline in C4D, divide the surface with knife and extrude resulting polygons as needed - but I hope to find faster and more automatic way of doing this. Just imagine that you have gray-scale city plan where height of the buildings reflected in the brightness value and you want to quickly build city based on this plan - what would be the most efficient way to do it in C4D?

You are right, no need for knife, since the whole thing can be done in Illustrator. From your example I kind of see the solution by using original image as texture for Shader Effector and Illustrator vectors (derived from the same image) as splines to extrude.

The possible problem comes from very random placements of gray-scale rectangles in question... but I guess I will need to experiment with this...

Thank you for your helpful answer!