20,000 yen or less: For fun. For beginners.
20,000-60,000 yen: For at-home practice or as a dedicated machine. You can roughly distinguish that dedicated machines have a phone output and can be used on the stage, and the ones for at-home use have a stereo mini output, which could cause clipping on the amplifier.
60,000-90,000 yen: Entry model stage synth. Sound pressure is comparable to higher-end models, but you can only control volume and filters to create sound.
90,000 to 150,000 yen: Middle class that can be used by professional musicians. The purpose is not as inclined as dedicated machines, but many are designed to be used for a specific purpose. You can make your "own sound". Sound making requires a certain level of knowledge and experience.
150,000 yen or more: High-end class. It is equipped with almost all the functions middle class models can have.