As you know carrying fuel externally is extremely inefficient. I forget the name of the ratio, but it is the ratio of amount of fuel carried internally for equivalent performance gain as the fuel carried externally. This number is between typically around 0.3 for very well designed fuel tanks if I remember correctly. To give you an example, LCA Mk1's fuel tanks are not optimally designed(they are doing so now). Adding the third centerline fuel tank hardly increases any range over the two wing EFTs. You would be surprised that this is not very uncommon. Rafale to its credit had one of the best designed EFT (thanks to work done on the Mirage 2000).
If the sortie is short, say less than two hours with A2A or light ground attack role, the Rafale would require considerably less fuel. I have to do the math there but in the range of 30%.
But the moment the Rafale has to put on the EFTs, the Sukhois advantage starts to climb. For example, with full internal fuel of 9.65 tons, the su-30 can stay aloft for over 4.5 hours. For the same kind of loiter time, the Rafale would need three 2000 ltr tanks. The above is not a guesstimate. You followed the Brazillian competition closely. You will also know this. That is the Rafale would need 4.8 tons of internal fuel + 3 * 0.8 * 2000 of external fuel which is also roughly 9.6 tons. You can give or take 10% here and there.
As I see it, the SFC of the M-88 is lower than the AL-31FN. Given that the SFC of 2 X M-88s will be much lower than that of 2 X AL-31FNs, the rate at which internal fuel will be consumed will be greater on a Su-30 than on a Rafale. So even if the Rafale and Su-30 weighed the same, the Su-30 is using up its internal fuel faster than Rafale. The Rafale carries less internal fuel but also weighs a lot less than a Su-30 (empty, normal, max, whichever weight).
For 80% of the sorties that a typical fighter squadron performs on a daily basis, this will be the case. The bulk of the sorties performed are training sorties that won't last longer than 60-70 minutes and EFTs are not needed for that duration mission. Having heard numerous fighter pilots' interviews, the amount of time it takes to plan for a mission, conduct briefing, carry out the mission and then undergo debrief, it is rarely possible to do more than 2 sorties, that too if sortie duration is 30-45 minutes only.
Over 40 years of service, that is a lot of sorties. And for 80% of those missions, the Rafale will consume far less fuel than a Su-30. JayS has come up with some ballpark figure which should be good enough for argument's sake. And you know that fuel is one the biggest contributors to the life cycle cost of any platform.
I agree that whenever there will be missions where duration is longer and EFTs will be required, the Su-30's huge internal fuel volume will mean it has an advantage in not needing EFTs. But, over a fleet of 250+ fighters, the Su-30 fleet's operational expenditure will be very high.