The BMW M5 Convertible never came to the production stage. I wouldn't be surprised if people haven't actually heard about it. So here is a quick history lesson for you.
In 1989, Motorsport engineers developed the M5 Convertible, which was set up to be displayed at the Geneva Motor Show. However, about a week before everything was to be a go, BMW felt that if the convertible series were to make it, people might demand the M5 non-convertible model, which would have led to a drop in sales for the 3 Series.
At this time we have a picture of what the prototypes looked like and to be honest, the car doesn't look that bad. It would have been a nice addition to the BMW family. The set price for the car would have been $50,000.
Source: bmw