>>How much cheaper is cheaper?

If they were trying to do this in 3 days like the Apollo missions, then doing it this way is a fraction of the cost. The issue is that a traditional Hohmann transfer orbit you need to use fuel to both accelerate the craft and deaccelerate it at the other end. This requires you to carry several times the payload weight in propellant.

By comparison Hakuto-R M1 needs only a tiny amount of propellant from ion thrusters, as the Sun's gravity is doing almost all the work.