No, but that doesn't magically make the development costs cheaper than a well-understood consumer machine of which literally billions have been mass-produced.
A prototype would only be a portion of the development costs. The private world would foot most of the bill, assuming that economically viable fusion reactors were demonstrated.
The millenium dome is 52 meters high on the inside and cost a more than a billion dollars and it's basically a giant tent. NASA's Space Power Facility is more the sort of thing you would need for a giant Farnsworth fusor. It's still only about forty meters high. I can't find exact costs for it, but I can guaranty it wasn't cheap and it's only a small fraction of the scale you're talking about.
These are prestige projects. They wouldn't build them, if the design were cheap. Another example, is the Khan Shatyr Entertainment Center in Astana, Kazakhstan. It's a 150m high tent structure which supposedly cost $400 million to produce.
This is what they're already building. I personally think it would be great if they could find the budget for a few different approaches.
This brings up an important point. The primary purpose of ITER is to immunize 34 national governments against accusations that they aren't doing publicly funded fusion research. That's the primary reason there's only one big approach rather than several different approaches.
NASA above is notorious for doing singleton missions in identifiable niches (like one Mars rover at a time, one space station at a time, or one space telescope at a time). That's because having one such thing is a great selling point for a US congresscritter, but having two or more is no more valuable. They don't even need to do very much (which is a serious current problem with the International Space Station).
There won't be a "few different approaches" unless someone in power has an actual interest in the research rather than the prestige of the research. For example, the US military has at least two different fusion research projects going because they want nuclear explosion data (for the National Ignition Laboratory) and a fusion power plant for naval ships (Polywell).
This also explains why air conditioning in Afghanistan can pull in a lot more money than fusion research. Losing a war is very dangerous to a political establishment. Ineffective fusion power research, that goes nowhere for decades, is not threatening.