The south of France is absolutely stunning. July and August, as mentioned, are hellishly busy; as every Parisian comes down south for the month. September and October are generally still sunny though less busy (and consequently cheaper). If you come in December or January you risk many of the seasonal shops and attractions being shut. April - May or Sept - Oct would be a good bet.
Eating-wise, make one meal a day very simple and very French: buy bread, cheese and cold meats from local shops (or a supermarket if you have to) and wash it down with a cold drink (wine if you're not driving, water if you are). This could be either lunch or dinner. There are plenty of picnic areas by the sides of roads where you can stop and eat. The toilets, however, may not be what you expect: motorway toilets tend to be a hole in the ground. You may wish to clench until you reach a supermarket or your hotel.
Breakfast: go and find a baker (France has hundreds) and eat pastries and get some coffee or hot chocolate from a cafe or orange juice from a supermarket. That will keep you going just fine. And it's cheaper than a hotel buffet.
In terms of saving money, consider either youth hostels (
http://www.fuaj.org/ ) or camping as opposed to hotels. Both would be cheaper, especially for three people. Look into budget hotels too - ibis budget, hotel F1 et cetera. These hotels can have 3 people in a standard room.
Monte Carlo, Nice, Cannes, St. Tropez.
- Why do I need so much money ? How to make it cheaper ?
You need so much money because you have listed the 4 most expensive places in France outside of Paris! There are plenty smaller places on the south coast worthy of a visit that would be cheaper. The further west you go, the cheaper it will be too - Toulon, Frejus etc are still coastal. As mentioned above, look a few kilometres inland for cheaper things.
And finally, as has been mentioned above, please learn a couple of French phrases. "Parlez-vous anglais?" [par-lay voo on-glay] is "Do you speak English?", and even that by itself makes a world of difference. The French do speak English, but there's a heady combination of shyness to speak a foreign language and pride in French as a spoken language that makes them reluctant to engage with anyone who comes blazing in, going "HELLO FRENCHIE. ME SPEAKA DA ENGLISH."