yeah the discount during RG is interesting. But it seems like they sell only a limited number of places during RG because a friend of mine couldn't buy any. So if you want to try that be careful.
Last year i bought my tickets during the charity day in RG. To give you an idea, i bought tickets for category one (i was sitting in the 5th row, facing the 1st players' chair when they go on court) and it was 25 euros each. The years before i bought them @ the store called La Fnac
(almost like Virgin Megastore, you find them everywhere here) and i paid 42 euros... and of course the seats i got were not as good as the ones i had last year. So you see the difference