Eurolines used to run a bus service from Barcelona to Nice on Wednesdays, Fridays and Sundays. It left at 10:30am and arrived at midnight.
On Mondays, Wednesdays and Saturdays the bus was an overnight journey, starting at 6:30pm arriving at 4:30am.

On other days and other times the bus only travelled as far as Marseilles.

Unfortunately I can't check out the current Eurolines timetable as I can't get the website to open with my Mac browsers. So no idea if this is still current (I suspect it is though)

Here's the timetable I used
http://www.eurolines.es/portal/pdfs/...a_toulouse.pdf

If the overnight times suit you and you sleep well on a bus it might not be such a bad option.