Well, running FGT wouldn't cause Saga any further amount of money, while most other options will cost Saga a lot so it's definitely cheaper in long run for Saga. The question is what about other parties involved.
And the Safga governor was comparing the development cost and time of a usable version of FGT Shinkansen with the construction cost and time of Full Shinkansen across Saga, it didn't mention operational cost, which I wrote in mistake in my previous post.
Also, the problems Japan is now facing with the FGT iirc isn't the change of gauge being too large, but is instead about the Spanish system cannot be downsized to fit in Japan's narrow gauge system, and hence they need to develop their own alternative system which now proved to be have less reliability, and such lower reliability resulting in the necessity of more frequent checkup and more frequent component replacement, together with the higher cost of FGT trains as well as their heavier weight making the cost of operation much more than regular Shinkansen.
And somehow, China now magically come up with a FGT train with mechanism similar to Spain's yet claim it'll be able to operate and change gauge between anything between 600mm to 1886mm, and can operate with speed up to 400km/h, which sound much more like magic than real technology but if they really can do this then as Toyo Keizai's article describe it'll be possible that we will see Chinese trains running in the Japanese system.