From Tokyo ?
Option 1 : Train only - 19830 yen
The downside is that the train cross to Shikoku at Okayama, so it's a detour.
Option 2 : Train to Kobe, Bus to Okayama, 15100 + 3300 = 18400yen
Option 3 : Train to Wakayama, Ferry yo Okayama, 16050 + 1900 = 17950 yen
Those prices are with shinkansen, it can be cheaper by slower trains, and also it's possible to do Tokyo to Okayama/Kobe/Wakayama part by bus, that is also cheaper.