The calculated distance (air line) between Roquebrune-Cap-Martin and Montpellier is approximately 180 miles respectively 290 kilometers.
By car or train, the actual journey to Montpellier is certainly longer, as only the direct route (as the crow flies) between Roquebrune-Cap-Martin and Montpellier has been calculated here.
Air line (approximately)
290 kilometers
157 nautical miles