[chromo]

The cost would vary for everyones own design and no 2 the same I'm sure. Otherwise I would say about the same cost as any oven the same as your max size, perhaps a few dollars more for some extra's.

Simply make all the floor panels interlock, put the heating elements in the floor. Run a power strip on the wall behind the oven, control the strip with the heater controls. Plug in the elements as needed to the power strip.

Make your floor, back, front, top, panels all the same size. Depth front to rear will be same as side panels, height will be same also.
Now snap together and plug in the sections you need for the size you want the oven to be.
How to seal all these panels to prevent lots of heat loss will be tricky. You would want the top sections laying on the front/back walls, perhaps channels to interlock, this is for support. With a modular oven like this then your only option is probably to load from the side, you need the front wall to support the top. You could run a solid beam, but you need one for about every size you might assemble, or you could bolt sections spliced together, too much work and time.