Hey! There are a couple of differences between the Octavox and the Quadravox. The Quadravox is a 4-voice diatonic pitch shifter based on technology from Eventide’s flagship H8000 effects processor with panning, delay, detune, and feedback to create beautiful harmonies and groovy sequences. Where the Octavox doubles that by having an 8-voice Diatonic pitch shifter. Another difference is that the Octavox allows for a MIDI in where you can use a MIDI controller and use the plugin live. 

As far as costs, the Octavox is currently $199. The Quadravox however is curently being offered at Reverb.com as a special for only $49.