The Sega Neptune was a Genesis and 32X combined into one console. Sega had admitted that the 32X was rather expensive and had problems, so combining it with a Genesis and selling it at a better price was a good idea. But by the time they had made a prototype, the 32X was nearing the end of its short life-span and the Sega Saturn was almost due for release, so the idea was scrapped as Sega believed there would not be much interest in it. If it had made it to store shelves, the Neptune would have cost around US$400.
Sega Mega Drive/Genesis Technical Specifications
Sega 32X Technical Specifications