Let's pretend a fusion plant is completely free to build, maintain, operate, and decommission.
50% of the cost of electricity is from transmission and distribution. Those costs aren't going anywhere.
So at most, you'll get a 50% savings. But you're not getting that either, because that fusion plant definitely wont be free.
ianaEE
Fusion reactors are mechanically simple and safe and can be made in tiny form factors, including engines. They are predicting container-sized "plants" that can power hundreds of thousands of homes. Fusion will heavily decentralize the grid (partially because of transmission costs).
Nothing is totally free of course, but fusion can do for energy what the internet did for distribution, which we can round to having made it free.