Original price unknown
The Yamaha AVX1 is a super rare MIDI-compatible AWM Voice Expander that comprise the expanded system of the HX Series Electone. An FM expander was also available: the Yamaha FVX1. The AVX1 is based on Yamaha's advanced wave memory (AWM) engine, which combines and filters sampled sounds to create voices, that was already being experimented with the TX1P and would later power the SY line of synthesizers.
The AVX1 doesn't have onboard samples, but uses external swappable ROM cards made specifically for the HX Series Electone. Up to eight of these cartridges can be inserted in a special drawer. Then it's possible to either play a single tone with 16 notes of polyphony or two of them layered with 8 notes of polyphony each. There are 16 performance slots available to save layer combinations.
Similarly to what is offered on the DMP7 and DMP11, the onboard mixers also has additional effects available: Stereo-Chorus, Stereo-Pan, Symphonic, Phaser, Flanger, Delay, Tremolo, Digital hall, Celeste and Wah.
Other than two German brochures there's no evidence of AVX1 expanders actually existing in the wild.