It varies per studio, engine version and sometimes per vocaloid.
For example, to buy Leon, Lola and Miriam is cheaper then buying Kaito and Meiko, despite all 5 being on the Vocaloid engine.
Kaito and Meiko cost more then Vocaloid 2 voicebanks in general.
Gakupo and Prima were the two most expensive vocals in the Vocaloid 2 era.
Take note that the price of Vocaloid 3 vocals are generally cheaper, because the engine is sold seperating to the vocal itself. You buy the engine only once, either on its own or with a vocaloid vocal. In Vocaloid and Vocaloid 2, the engine was sold with the vocal, pushing the price up much higher.
Vocaloid is a professionals' software, thus it is not cheap,with the price usually above $100 for just 1 vocal and to buy ll Vocaloid and Vocaloid2 vocals would cost you more then $2000+.