But the point is that these structures are essentially _tents_
And my point is that these are essentially status projects. Well, the NASA one is a reused status project. Spending a lot more money than you have to is part of the project.
The same goes for pyramid shaped objects. Nobody builds a cheap pyramid just because even though it'd be relatively easy to do with a few earthmovers and other massive construction equipment. It's, for example, a place of worship, monument to your life, and/or a giant casino. Using historical examples for price estimates would give some amusing claims. "You would need 10,000 slaves to build a pyramid."
For example, I believe an inflatable structure of the appropriate scale with a medium vacuum in the center and properly anchored to the ground (or perhaps rather the inside of an abandoned open copper mine) could be had for low tens of millions of dollars (the inflatable components of the outer shell would be moderately over-pressurized cone-shaped wedges which would need to resist one atmosphere of pressure and wind loading with appropriate factor of safety). That includes building of smaller structures to get the many design issues worked out. That's not quite good enough a vacuum, but it's getting there.
High low Earth orbit in space has a similar density and composition to what a Farnsworth fusor would use, so you could just build a bare fusor in space, say 600 km up and use atmosphere (which would be mostly hydrogen and helium isotopes) at that point and see what happens. I haven't given it much thought past that point, the interactions with higher atomic weight atoms, Earth's magnetic or gravity fields, or the lower Van Allen belt might render it useless for most proper fusion research. But I think you could do such a thing for under a billion dollars today - even with today's high launch costs.