Lemme see the math in this...
It costs ~$20 to fill up my 'Tour.
With this Mod, I could get up to 3 times the current mileage. I average 20MPG, it's a bit low for some reason - I haven't figured out why, but it's a nice round number for us to use. With this "Mod" I could average 60MPG.
At 20MPG, it costs roughly 9 cents/mile. With this mod it's 3 cents/mile.
Do the math, and it will take roughly 65,000 miles of travel before I even break even on my investment, not counting the cost of capital, which is pretty high for something that will take me about 4 years to break-even on.
It also doesn't include cost of labor, and I'm sure this is not an easy thing to install. Anything requiring removing the entire engine, taking it apart, and drilling holes in the block, all by a "specialized mechanic" is going to cost a considerable amount -- Well over $1000, I'm sure.
Cost of extra maintanence -- extra parts means more maintanence, especially since you're modifying parts that weren't meant to be modified.
Cost of repairing if things break -- You can't just walk into a Ford dealership or AutoZone and ask for a "Plasma Coil Coolant Hose" or something.
So, figuring in all those things, and you're talking at least 150,000 miles, about 10 years for the average person, before I would even break even on my investment, let alone see a profit. And that's provided that the modification even works that long -- no one's installed one of these and used it for that long before to know if it even can take the abuse.
Even if gas prices went, and stayed, at $2/gallon, it would still take 136,000 miles, about 8 years or so for most people, to break even. Most people wouldn't break even the entire time they had the car.
And it is for this reason that plasma technology never took off as a viable alternative to traditonal gasoline engines. Not that it wasn't looked at seriously by automotive companies, because it was some time ago. But they determined that gas prices would have to exceed $5.00/gallon before the economics of it would even pay off -- and that's the economics of mass-production costs. That number gets even higher since the costs of the conversion are so much higher on the small-scale that this technology is being sold at.