Discount airline Vueling flies Nice-Barcelona on Tuesdays, Thursdays, Fridays, and Sundays. The farther ahead you book, the better the price.
If that doesn't fit into your timetable, then train is the other option. The fastest daytime train takes about 10 hours with one change. You could buy these at tgv-europe.com and get a good discount....keep your country set to Great Britain.
From Madrid to Paris, flying is the best way to go. Assuming you want to go to Charles de Gaulle airport, you can check discount airlines Vueling and Easyjet. Air Europa also flies to Orly airport. Be careful with luggage allowanced and charges, and check-in times. Check the "big guys" on this route as well....Air France, Iberia, Spanair. Many times, when they have competition from discount airlines, they'll offer comparable rates, but may have better flight times and luggage allowances.