Just in case there was anyone still wondering how this works IF you were NOT here for Halloween last year....
When you unlock the Victorian spaceship GOO prize, you automatically get Kang with it.
After you unlock the Ultrahouse 2 GOO prize and complete the Homer task that goes with it, you unlock the Ray Gun in the store. Kodos will start to fly around your town in a UFO. Once you buy the Ray Gun (150 D), you can shoot down Kodos and he will crash into your Ray Gun (which then unlocks Kodos as a character). A second Ray Gun is then put into your inventory, which can also be placed in your Springfield (and fired!)
So for those new to Halloween this year: you get Kang for free via GOO, and Kodos (premium character) + 1 wrecked Ray Gun + 1 new Ray Gun + 3.75% multiplier for 150 donuts.