Bugatti (owned by Volkswagen) is planning a Veyron Targa. This is pretty much all on information VW is giving away at this point in time about the super convertible car.
The currently available Bugatti Veyron has 1001hp and reaches a top speed of 407 km/h (~252mph). Whether or not the Veyron Targa can reach these stunning performance marks is unclear.
Most convertible versions of cars are more expensive than the closed versions. It is safe to assume VW will charge more for the Veyron Targa than it does for the Veyron. For the customers who can afford a Veyron (~$1.2m) price does not matter.
Via Bild Zeitung (German). See also the Bugatti Veyron site. The site features a Veyron 16.4 configurator, just in case you like to order one.