If you think about it, the butt end of every MKK is actually from the Su-35. We're talking of the tail structure, the elevator, the boom, right from back second half of the aircraft.
Sukhoi uses names like Su-30 and Su-35 like marketing gimmicks. For all you know, anything can be called Su-35BM or Su-30MK4. But it does sound a lot cooler with a higher number. The RuAF alone resisted all numbering above Su-27 until recently. All Su-30, 32, 33, 34, and 35 are officially given -27 plus a letter digit.
I still smile at Sukhoi's marketing gimmicks. You get a software upgrade on the Su-30MKK and it deserves a "2" digit? When the F-16Cs get their software upgrades, they're not called F-16C1, C2, C3 or so forth. I'm pretty sure Su-30MK2 sounds nice on the printed contract as an excuse to add some surcharge on it.
I always have this belief that "Su-30" came out out because the name "Su-27" sounds below "MiG-29". But 30 is above 29. The first Su-30s by the way, are nothing more than Su-27UBs with a retractable IFR probe and a new peer to peer datalink system that extends data interaction from 4 planes to 16. The Su-27SKM and the Su-27SMK before it, are far more functional multirole planes the equivalent of an Su-30MKK in single seater but again the RuAF which were eyeing the planes, won't accept the post Su-27 jargon.
Can't have the "31" number because MiG got it. Because MiG failed to reserve the 33 number, Sukhoi quickly grabbed 32, 33, 34, 35 and 37. MiG can always upgrade their 29, with SMT, M, M2, etc,. but it will always be a "29". Sukhoi has won for now, the mind game in number positioning. Simonov may have grown up in a communist country, but he sure learned Western style marketing gimmicks quickly.
Understandably, once the number goes up, you are going to expect them to "charge' for it too. Su-35 won't be cheap but then again, when China started negotiating for the Su-30MKK, it had less than a hundred billion in foreign reserves. Now it has well over 10X as much.